Expandable communication system with data flow control

ABSTRACT

A communication system including one or more host adapters connected to a host computer, each adapter having multiple serial communication ports for transferring data between the computer and several TTY devices. Several of the adapter&#39;s serial ports include a high speed serial link for communicating with a data concentrator. The adapter automatically detects the presence of a concentrator connected to a switchable port and switches to the high speed link. Each concentrator includes multiple serial ports for communicating with TTY devices, and a high speed serial link for communicating with the adapter&#39;s high speed link. The concentrators allow more than one TTY device to share a single adapter serial port. Data from all of the TTY devices is accumulated into an adapter data buffer during a configurable time period or until a certain amount of data is accumulated, at which time the adapter interrupts the computer and transfers the accumulated data to the computer in one transfer operation. Likewise, the computer accumulates data for the TTY devices and transfers this data to the adapter in one transfer operation. Communication between the adapters and concentrators through the high speed serial communication link is implemented using a small fixed-size addressed packet to achieve a low overhead, high performance communications protocol. Each high speed link between an adapter and a concentrator is implemented to provide inherent flow control of data. The concentrators include a fail safe global flow control mechanism to prevent overflow of data from the TTY devices.

This is a continuation of co-pending application Ser. No. 07/771,169,filed on Oct. 3, 1991.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application relates to U.S. patent application Ser. No.771,143, filed Oct. 3, 1991, entitled "Expandable Communication SystemUsing Data Concentration"; U.S. patent application Ser. No. 770,617,filed on Oct. 3, 1991, entitled "Data Transfer System Between a Computerand a Host Adapter Using Multiple Arrays"; U.S. patent application Ser.No. 771,159, filed on Oct. 3, 1991, entitled "Data ConcentrationInterface for Configuration of Multiple TTY Devices"; U.S. patentapplication Ser. No. 771,121, filed Oct. 3, 1991, entitled "ExpandableCommunication System with Automatic Data Concentrator Detection"; andU.S. patent application Ser. No. 770,600, filed Oct. 3, 1991, entitled"Data Transfer System with Variable Data Buffer Size and ProgrammableInterrupt Frequency".

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of digital data processing systems,and more particularly relates. to communication of information betweencomponents in such systems.

2. Background of the Invention

In the field of digital data processing systems, as the computercapabilities of data processing devices have increased, both in terms ofspeed, efficiency, and complexity, the number of users or applicationswhich may be concurrently supported by a single central processingdevice has also increased.

Often, the number of remote devices in a system exceeds the number ofseparate input/output (I/O) ports of the central processing device. Inthis case, a multiplexing communications system may be implemented whichallows multiple remote devices to share a common one of the centraldevice's I/O ports.

Remote or "peripheral" devices, such as user terminals, printers,modems, data storage devices, data acquisition devices, and the like,must frequently exchange information with a central processing unit, andas the number of remote devices increases, such communication of databetween peripheral devices and a central unit must be highly efficient.The communications system must also allow the central device toselectively exchange data with any one of the remote devices, and eachremote device must have a unique identity as seen by the central device.Accordingly, any communications scheme for multiplexed systems mustinclude not only the hardware components for implementing the actualdata link, but also a communications protocol for use with the hardware,for allowing data transferred on the link to be associated with aparticular sender or receiver.

Maintaining the identity of devices and data in a multiplexed system canbe accomplished in a number of ways. One method involves the assignmentof a unique identification code to each device in the system, includingidentification of the sender and/or receiver of data within the dataitself as it is transmitted along the shared data link. An example ofthis is the so-called address/packet protocol, wherein all informationtransmitted on the shared data link is of a standard format whichincludes device identification fields. Packets of data transmitted on adata link that is shared by multiple devices could be received by alldevices at once; each device could then decode the identification fieldof the incoming packets to determine whether the data is intended forit.

In another type of multiplexing, called Time Division Multiplexing(TDM), the identity of data is determined by the time of itstransmission on the shared medium. Each device sharing a common datalink is allocated, in a regular and cyclical fashion, a fixed period oftime during which it has access to the data link. If a first deviceneeds to send data, it must wait until its allocated time slot beforesending it. The identity of the sending device is thus implicitly knownto the receiving device according to the time of transmission.

Alternately, access to a shared data link can be multiplexed on ademand, or First-Come-First-Served (FCFS) basis, in which a device isgranted access to the data link not in a cyclical fashion, but accordingto its needs. Often, demand multiplexing is implemented through the useof a separate multiplexor unit, which receives requests for access tothe shared data link from multiple devices, and allocates accessaccording to a FCFS algorithm. With demand multiplexing, the multiplexorlogic must be capable of informing each device of the source of datatransmitted on the shared link, since time slices are not allocated in arepeating, cyclical manner.

Additionally, variations of demand multiplexing can be implementedwherein the FCFS algorithm is replaced by a weighted algorithm whichconsiders other factors, such as varying levels of device priority, inallocating access to the multiplexed data link. In any non-symmetricalimplementation in which one device may have temporary or permanentpriority over another, care must be taken that low priority devices arenot prevented entirely from gaining access to the shared data link byhigher priority devices. Typically it is the duty of the multiplexinglogic (either software or hardware) to ensure equity or near-equity ofaccess to a shared communication medium.

The aforementioned methods of multiplexing each have theirdisadvantages. Multiplexing schemes typically require substantialhardware support for allocating access to the shared link in anefficient and equitable manner. In pure TDM schemes, inefficiency isunavoidably introduced if any devices are idle, because the shared linkwould go unused during an idle device's time slot. Though no bandwidthmay be lost in demand and other priority-based access schemes, thesemethods may also suffer from inefficiency if the multiplexing algorithmallowed one very active device to dominate over all others. Inaddress/packet protocol multiplexing, on the other hand, eachparticipating device must provide hardware or software for"de-packetizing" all incoming data packets to determine the intendedrecipient. Hardware or software must also be provided in each device for"packetizing" all outgoing data. The processing overhead forpacketization and depacketization is further increased if the packetsize is allowed to be variable under a given protocol.

The addition of devices or user terminals to multi-user systems oftenrequires a great deal of system reconfiguration. As the limits of thesystem's communications hardware are reached, new hardware must beadded, or the existing hardware must be replaced by new, more denselypacked hardware. Such hardware changes typically involve reconfigurationof the system software or firmware to accommodate the new hardware.

SUMMARY OF THE INVENTION

The present invention provides a communications system that can bereconfigured with respect to the number of remote devices withoutrequiring alteration of communication software in the central device.The communications system includes hardware and an associatedcommunications protocol that affords efficient and equitablecommunication between remote devices and a central device, and that iseconomical to implement even when remote devices are located relativelyfar from the host. The communications system provides the capability tomodify the communications configuration depending upon the needs of thesystem having various numbers of devices with various speeds andcommunication protocols. The system allows remote devices to be locatedvery far from a central device, without introducing significantdegradation of overall system performance.

The foregoing and other features of the present invention are realizedin a communications system comprising one or more host adapters, eachequipped with multiple serial communication ports capable of exchangingdata in serial form with one or more remote devices. Each host adapteris provided with an additional communications interface, which isappropriate for exchanging data with a host processing unit orcommunications processor. Several of the serial communication ports onthe host adapter include switching means to enable a high speed serialcommunication link on these ports. When expansion is desired, a dataconcentrator may be connected to any of the switchable ports of the hostadapter instead of a device. The host adapter is capable of detectingthe data concentrator through a software command or an auto-detectionmechanism, and enabling the high speed communications link to interfacewith the data concentrator.

The data concentrators are also provided with multiple serialcommunications ports, and are capable of exchanging data in serial formwith one or more devices. A high speed serial communications port allowsa concentrator to be coupled to any one of the switchable ports of thehost adapter in place of a single device. In this manner, concentratorsperform a multiplexing function, and permit more than one device toshare a single serial port of the host adapter.

Communication between the host processing unit (host) and the hostadapters is enhanced by two software packages compatible with the host'soperating system. The preferred embodiment runs in the UNIX environment,but the present invention can be implemented on other operating systemsas well. A device driver is provided to execute on the host andinterfaces directly with the host adapter. A command protocol betweenthe device driver and the adapter gives the device driver control overthe operations of the adapter. The device driver can command the adapterto initialize data structures, to provide status information, to set updirect memory access (DMA) and interrupt channels, to configure all ofthe remote devices coupled through the host adapter and to do many otherfunctions necessary to establish communications between the host and theremote devices through the adapter. The device driver can also retrievefrom the adapter the number of remote devices as well as a uniqueidentifier for each device. These commands are passed through a set ofmailbox registers provided on the adapter and mapped into theinput/output (I/O) space of the host.

The device drivers work in conjunction with a software configuration anddatabase management utility, as well as with the host adapters, toconfigure each of the remote devices coupled through any of the hostadapters. The management utility configures the operating system of thehost which builds and manages a cross-reference data file comprising theunique identity and configuration of each remote device. The data fileis maintained on the host system, such as on a hard disk of the hostcomputer. The device drivers use the cross-reference data file toidentify and to establish the configurations of the remote devices. Thedevice drivers, however, may not directly access the devices, butinstead send the configuration commands to the host adapter pursuant toa command protocol established between the host and the host adapter.The host adapter follows the commands and configures each device eitherdirectly, or indirectly through the data concentrators. The host adapterassigns unique identifiers to each remote device, and these identifiersare passed back to the host and are maintained in the cross-referencedata file.

Data transfer between the host and host adapters is implemented in amanner which reduces overhead, simplifies database structures, increasesefficiency, and reduces the processing requirements of the host. A setof database I/O buffers in the memory of the host facilitate datatransfer. A contents buffer, maintained in the host's I/O space andcopied in the host adapter's RAM, is used to identify each byte of datain the database buffer. Also, each host adapter contains enough localRAM to interface with these database buffers. Data from all of theremote devices, whether connected directly to the host adapter orcoupled through a data concentrator, is combined and stored on the hostadapter's local RAM. When a certain amount of data is compiled, or,after a certain configurable amount of time has passed, the data in theRAM is transferred to the host's database buffers in one transferoperation. In this manner, multiple bytes of data are sent in onetransfer operation from multiple devices, rather than one operation foreach byte or for each device. The contents buffer is used to identifyeach byte in the database buffer, so that no particular order need bemaintained. In a similar manner, the host builds up the database and thedata is transferred to the local RAM of the adapter in one transferoperation. Furthermore, the adapters support direct memory access (DMA)such that the adapter can control the data transfer to the host to freeup the host for other tasks. These methods allow reduced overhead andreduced processing requirements for the host which frees much of thehost processor's time for other tasks.

The present embodiment supports up to four adapters to be installed onthe host. If more than one adapter is installed in the host system,pairs of adapters may communicate small amounts of data and controlinformation through an interboard communication connector located oneach adapter. The adapters may be configured to operate independently orin pairs. Each adapter or pair of adapters requires a system (host)interrupt request (IRQ) level and a DMA channel. Pairing of adaptersallows sharing of system resources of an IRQ level and a DMA channel.Sharing of the system interrupt between pairs of adapters is arbitratedin software via the communications channel provided by the interboardcable.

The high speed serial communication link between a host adapter and adata concentrator is presently implemented with high speedreceiver/transmitters (HSRT's) capable of speeds of up to 600 kbits persecond. A command protocol is also established between the host adapterand the data concentrators. In this manner, the host adapter can controlthe operations of the data concentrators and the communicationconfiguration of each remote device attached to the data concentrators.The data concentrators can also initiate a fail-safe global flow controlstate to prevent overflow of data-coming in from all the remote devicesconnected to that particular data concentrator. All commands, includingflow control commands, and data transfer share the same high speed link.For this purpose, the present invention uses a fixed-size, relativelysmall addressed packet to achieve a low-overhead, high performanceprotocol for demand multiplexing. The packet comprises a frame-typefield, an address field and a data field. The frame-type field indicateswhether the packet contains a command or data. The address fieldidentifies the specific remote device if the information is sent from,or to be sent to, that remote device. The contents of the data fielddepends upon the frame-type field and either contains data or commandspecific parameters. If digital information is traveling from the hostadapter to the data concentrator, the adapter assembles the packet andthe data concentrator disassembles the packet and transfers data to theremote devices, if necessary. In a similar manner, the data concentratorassembles the packet containing digital information traveling from theconcentrator to the adapter, and the adapter disassembles the packet andstores the information in its local RAM, if necessary. It will beappreciated that the data concentrators require little RAM due to thevery small fixed-size communication protocol packet.

The high speed interface also comprises unique inherent flow control ofdigital information. In the present embodiment, each switchable channelon the host adapter, as well as each data concentrator, includes an HSRTto implement a full duplex synchronous high speed communications linkfor transferring digital information. The link includes two clock signallines, a transmit and a receive signal line. The receiving HSRT providesthe clock to the transmitting HSRT, to latch the data into the receivingHSRT across the data link. In this manner, the receiver, whether it is adata concentrator or a host adapter, controls when data is accepted.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other novel features believed characteristic of thecommunications system of the present invention are set forth in theappended claims. The invention itself, however, as well as additionalfeatures and advantages thereof, will be best understood by reference toa detailed description of a specific embodiment which follows, when readin conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a multi-terminal system of the prior art;

FIG. 2 is a block diagram of an expandable, multi-terminal system inaccordance with one embodiment of the present invention;

FIG. 3 is a block diagram of the host adapter according to the presentinvention as shown in the system of FIG. 2;

FIG. 4 is a block diagram of the data concentrator in the system of FIG.2;

FIG. 5 is a flow chart illustrating the sequence of events which occurwhen the host adapter processes commands from the host computer;

FIG. 6 is a state diagram illustrating the operation of the receiverstate machine of an HSRT located on a data concentrator;

FIG. 7 is a schematic diagram of the double buffered receiver sectionwithin each HSRT;

FIG. 7A is a state diagram illustrating the operation of the receiverbuffer controller state machine which controls the operation of thedouble buffered receiver section of FIG. 7;

FIG. 8 is a state diagram illustrating the operation of the transmitterstate machine of an HSRT located on a host adapter;

FIG. 9 is a schematic diagram of the double buffered transmitter sectionwithin each HSRT; and

FIG. 9A is a state diagram illustrating the operation of the transmitterbuffer controller state machine which controls the operation of thedouble buffered transmitter section of FIG. 9.

DETAILED DESCRIPTION OF A SPECIFIC EMBODIMENT TABLE OF CONTENTS

Description of System Components

Description of the Host Adapter 18'

Configuration of the Host Adapter 18'

Communication Between the Host CPU 10 and the Host Adapter 18'

The Host Adapter 18' Functions

Automatic Data Concentrator 28 Detection

Host CPU 10 to Host Adapter 18' Command Protocol

Host Adapter 18' Configuration and Status Commands

TTY Port Commands

Direct High-Speed Channel Commands

Miscellaneous Commands

Device I/O Data Transfer Protocol

Description of the Data Concentrator 28

Addressed Packet Protocol

Flow Control

The Host Adapter 18' and the Data Concentrator

Addressed Packet Protocol

Referring to FIG. 1, a block diagram of a multi-terminal data processingsystem of the prior art is shown, comprising a host central processingunit (CPU) 10, remote terminals 12, 14, and 16, and an interface logicunit or host adapter 18.

Generally speaking, the function of the host adapter 18 is to establishcommunications between the host CPU 10 and some number of remotedevices. The host adapter 18 is necessary because often the host CPU 10and a remote device, such as the terminal 12, may not communicatedirectly. As shown in FIG. 1, for example, the host CPU 10 communicateswith peripheral devices via a parallel communications link, inparticular, a system bus or data link 20 of the host CPU 10, while theremote devices 12, 14, and 16 communicate via the serial links 22, 24,and 26, respectively. Thus, the host adapter 18 is required to translatedata from the parallel bus protocol of the host CPU 10 into the serialprotocol recognized by the terminals 12, 14, and 16, and similarly totranslate serial data from the terminals 12, 14, and 16 into theparallel data recognized by the host CPU 10.

Host adapters, such as the host adapter 18, are widely used in knowndata processing systems. Through the use of an appropriate host adapter,a remote device conforming to virtually any communications protocol maybe coupled to any processor which may itself conform to any of variousprotocols. Host adapters may also perform functions other than logicaltranslation; in FIG. 1, for example, the host adapter 18 additionallyperforms the role of a multiplexor, allowing the three terminals 12, 14,and 16 to be coupled to the data link 20 of the host CPU 10, in atree-configuration.

Since data from each of the three terminals 12, 14, and 16 is receivedby the host CPU 10 via the data link 20, the communications system mustprovide some mechanism for preserving the unique identity of eachterminal as seen by the host CPU 10. Accordingly, a communicationsprotocol may be adopted which allows the host adapter 18 to identify thesource and/or destination of data as it is exchanged between the devicesand the host. For example, in one embodiment the host adapter 18includes a three-bit field with all data routed to the host CPU 10identifying the originating terminal of that data.

As shown, the system of FIG. 1 can include at most eight remoteterminals, one for each of the serial ports SO through $7 of the hostadapter 18. If additional terminals were to be added to the system ofFIG. 1, extensive hardware reconfiguration would be required. Inparticular, one known way to add another terminal is to replace the hostadapter 18 with a host adapter having additional serial ports; thiscould also require the communications protocol used between the host CPU10 and the host adapter 18 to be modified, since additional bits may berequired to provide for the unique identification of terminals coupledto the host adapter 18. Additionally, software for the host CPU 10 mustbe modified to allow it to recognize the existence of the additionalterminal or terminals thus accessible via the data link 20.

DESCRIPTION OF SYSTEM COMPONENTS

Referring now to FIG. 2, a system in accordance with one embodiment ofthe present invention is shown in simplified block form, whereincomponents that are the same as in FIG. 1 have retained identicalreference numerals.

In FIG. 2, a host adapter 18' is used to establish communication betweenthe host CPU 10 and a plurality of remote terminals 12, 14, 16. Theinterconnection scheme shown in FIG. 2 may be characterized as amultiplexed, multi-level tree-configuration, the multiplexing beingimplemented in two stages: First, the host adapter 18' permits serialports S1 through S7 to be coupled to the host CPU 10 via the shared datalink 20, in a manner similar to that of FIG. 1. In the system of FIG. 2,however, a data concentrator 28 allows up to eight device ports D0through D7 to be coupled to the host adapter 18' via a single, sharedserial link 30 to serial port SO of the host adapter 18'. Four of theserial ports SO through S7 of the host adapter 18', for example ports SOthrough S3, are capable of receiving input from a data concentrator 28,allowing up to 36 serial devices to be connected to the host CPU 10 viathe host adapter 18'. As many as four host adapter boards similar to thehost adapter 18' may be installed in a typical computer system, allowingup to 144 serial devices to be interfaced with the host CPU 10.

DESCRIPTION OF THE HOST ADAPTER 18'

Referring now to FIG. 3, a block diagram showing the primary componentsof a particular implementation of the host adapter 18' of FIG. 2 isprovided. It is to be understood that the particular implementation ofthe host adapter 18' shown in FIG. 3 is provided for illustrativepurposes only, and is not intended to restrict the scope of the presentinvention to a specific implementation.

As shown in FIG. 3, the host adapter 18' comprises: a microprocessor,referred to as host adapter CPU 40, and microprocessor support andconfiguration logic 41; a 64 kbyte block of random access memory (RAM)42; a set of general purpose 16-bit input/output (I/O) registers andstatus registers designated collectively as mail registers 43; interruptrequest logic (IRQ) 49; an 8 kbyte block of read only memory (ROM) 44;an octal universal asynchronous receiver/transmitter (octal UART) 46;four high speed receiver/transmitters (HSRT's) 48, 50, 52, and 54; busmaster control logic 56 which includes logic for direct memory access(DMA); eight RS-232 serial line drivers designated collectively as 58;four high speed RS-422 serial line drivers designated collectively as60; interboard communications logic and connector 47; concentratorselect logic 45; and relays 68a, 68b, 68c and 68d. In the presentlydisclosed embodiment of the invention, the bus master control logic 56,the four HSRT's 48, 50, 52, and 54, the microprocessor support logic 41,the mail registers 43 and the interrupt request logic (IRQ) 49 areconventionally implemented in a custom application specific integratedcircuit (ASIC).

The bus master control logic 56 enables the host adapter CPU 40 tooriginate and accomplish DMA transfers into and out of host memoryindependent of the host CPU 10. The HSRT's 48, 50, 52, and 54 providecommunications links to data concentrators such as the data concentrator28 at up to 600 kbits per second. The high speed communications channels30 feature an internal flow control mechanism to prevent data overrunbetween the host adapter 18' and data concentrators such as the dataconcentrator 28. Each of these components shall be hereinafter describedin greater detail.

In the preferred embodiment of the invention, the host adapter 18' isdesigned to be received into an expansion slot of a host computer, suchas the industry standard sixteen bit expansion slot of an IBM PC/AT orthe like. Alternatively, the host adapter 18' may be designed to bereceived in an expansion slot conforming to the Extended IndustryStandard Architecture (EISA) or Micro Channel Architecture (MCA)conventions, or may be otherwise connected to the host computer. Asanother alternative, many or most of the circuit comprising the hostadapter 18' may be built into a motherboard or similar circuit board ofa host computer.

Once installed in a computer, the host adapter 18' provides a highdensity 68-pin connector 62 at the back-plane mounting bracket of thehost computer. An external terminal connection fan-out device 64connects to the connector 62, providing eight RJ-45 modular jacks66a-66h, and permitting attachment of up to eight serial devices to thehost adapter 18' via readily available cables, connectors, and adapters.As will be hereinafter described, four of the eight modular jacks, i.e.jacks 66a-66d in the fan-out device 64 (corresponding to serial ports SOthrough S3, respectively, of the host adapter 18') may be used toconnect up to four device concentrators, such as the data concentrator28, to the host adapter 18'.

Again referring to FIG. 3, the host adapter CPU 40 is preferably ageneral purpose, high performance sixteen-bit CPU, such as a 16 MHz NECuPD70136 device, manufactured by NEC Corp. A microprocessor faster thanthe 16 MHz NEC CPU may be employed if higher throughput capability isdesired.

The host adapter 18' includes the 8k ROM 44 which contains the boot codefor the host adapter CPU 40. The position of the ROM 44 in the memoryspace of the host adapter CPU 40 is determined by the host adapter CPU40's reset entry point. After reset, the host adapter CPU 40 beginsexecution at address FFFF0h, and from there it executes a jump toinitialization and consistency check routines in the ROM 44. The ROM 44may be a relatively low-speed device, so its resident control program isthen copied in a conventional manner into a portion of the RAM 42 forfaster execution. The resident control program provides the basicterminal communications capabilities; however, it also supports thedownloading of program patches and additions, as well as completelydistinct replacement control programs.

In the present embodiment of the invention, the octal UART 46 is acommercially available CD-180 device, manufactured by Cirrus Logic,which supports asynchronous communications at speeds up to 38.4K bitsper second. The internal architecture of the UART 46 includes threereduced instruction set (RISC) processors for supporting the eightasynchronous lines' bit processing, as well as character buffering, flowcontrol, and other character processing capabilities. In addition, theUART 46 provides character receive and transmit buffers for each of theeight asynchronous communications channels.

The host adapter 18' communicates with the host CPU 10 via the generalpurpose mail registers 43 which are conventionally mapped into sixteenbytes of the I/O address space of the host CPU 10 and sixteen bytes ofthe memory space of the adapter CPU 40. The host adapter 18' consumes nohost RAM address space, so that unlike many multiport adapters of theprior art, the host adapter 18' is not a shared memory device.

If more than one host adapter, such as the host adapter 18',is installedin a system, pairs of boards may communicate small amounts of data andcontrol information through the interboard communication connector 47.Up to four host adapter boards can be connected to the host computer,and each board has a board identification number (BID) from 1-4. The BIDis determined by a four position slide switch located on each board. Thebits 0-2 of memory location 10015h of the memory map of each hostadapter CPU (described herein, see Table 10), contains a 3-bit codewhich corresponds to the BID, which identifies that board's BID to thehost adapter CPU.

The odd BID numbered host adapter boards (1 and 3) can be configured asparent boards, and the even BID numbered host adapter boards (2 and 4)can be configured as a child board. Resources can be shared betweenboards 1 and 2 and between boards 3 and 4 by connecting these boardstogether with a ribbon cable (not shown) attached to the interboardcommunication connector 47 of each host adapter board. The paired boardis accessed through the interboard communication registers at addresses10100h and 10180h of the host adapter CPU memory map. Each board canreceive 4 bits of data from, and send 4 bits of data to its pairedboard.

When so paired, the host adapter boards share an IRQ request and a DMAchannel thus conserving system resources. The parent board has directaccess to the DMA and IRQ channels on the ISA or EISA bus, while thechild board does not. When the child board needs a DMA access or IRQrequest, it sends signals to the parent board over the ribbon cable. Theparent board, when idle, sends the request to the host system, andpasses the host acknowledge from the host back to the child board.

Whenever the present owner (parent or child) finishes with its DMA cycleor has to give up the bus because of a refresh timeout, the next DMAcycle transfer is performed by the other requestor in the pair. Forexample, if the parent is currently doing its transfer over the DMAchannel and must discontinue because of a refresh cycle, then the nextDMA transfer will be performed by the child. Then, when the child istimed out by the refresh cycle, the next DMA transfer will be performedby the parent. Furthermore, if the parent finishes its transfer withoutbeing timed out by a refresh cycle, the child is granted the bus so thatit continues with its transfer. Note that the bus is not surrendered tothe host in this last transaction between the parent and the child. If,on the other hand, there are no requests from the `other board` and theboard that has the bus finishes one transfer without a timeout, itcontinues with the second transfer request from its holding registerswithout ever giving up the bus.

As described above, host adapters such as the host adapter 18' may beconfigured to operate independently or in pairs. Each host adapter orpair of host adapters requires a system interrupt level and a DMAchannel. The host I/O base address, interrupt request (IRQ) level andDMA channel are software selectable. The IRQ level may be selected fromIRQ's 10, 11, 3, 5, or none. The DMA channel may be selected fromchannels 3, 5, 6, or 7.

Communication of data between the host CPU 10 and the host adapter 18'occurs in a manner specific to the type of host CPU 10. Data is readfrom or written to the host CPU bus 20 by the host adapter 18' undercontrol of the bus master control logic 56 and the host adapter CPU 40.For example, when the host CPU 10 desires to send data to the hostadapter 18' the host CPU 10 will drive an appropriate I/O address on theaddress lines of the host bus 20. This address is detected by thesupport logic 41 and the data is accepted by the host adapter 18'.Conversely, when the host adapter 18' desires to send data to the hostCPU 10, the bus master control logic 56 will obtain control of the hostbus 20, drive the appropriate address signals on the address lines ofthe host bus 20, and subsequently provide the data on the data lines ofthe host bus 20.

The host adapter 18' communicates with asynchronous serial devices viaindustry standard RS-232 line drivers 58 coupled to the backplaneconnector 62. Alternatively, the host adapter 18' may communicate withthe data concentrators 28 via industry standard RS-422 line drivers 60coupled to the backplane connector 62. When the fanout device 64 isconnected to the backplane connector 62, modular jacks 66e through 66hprovide an RS-232 asynchronous, serial interface to which anyRS-232-compatible device may be connected. Modular jacks 66a through 66deach selectively provide either an RS-232 asynchronous connection or anRS-422 connection, to which an RS-422-compatible synchronous device maybe connected. The relays 68a through 68d perform a switching functioncontrolled by concentrator select logic 45 to provide either the RS-422or RS-232 interfaces at the modular connectors 66a through 66d,respectively, of the fanout device 64.

A data concentrator enable register is located at address 10200h of thememory map of the host adapter CPU 40 (see Table 10, below). A "1"written to any one of the four lower bits enables the corresponding dataconcentrator attached at the corresponding port S0-S3 of the hostadapter 18'. The corresponding one of the relays 68a-68d is enabled bythe concentrator select logic 45, thus disabling the correspondingRS-232 line driver 58 for that port. A "0" written to the concentratorenable register bit will enable the corresponding RS-232 58 for thatport.

CONFIGURATION OF THE HOST ADAPTER 18'

Configuration of the host adapter 18' is performed by the host adapterCPU 40 as commanded by the host CPU 10. For Industry StandardArchitecture (ISA) systems, the board number selector switch (not shownin FIG. 3) must be set to uniquely identify each board in a system inorder to configure the I/O base address.

The manner by which the I/O base address for the host adapter 18' isconfigured depends on whether the host adapter 18' is installed in anISA system or an Extended ISA (EISA) system. If the host adapter 18'detects that it resides in an EISA system, the I/O base address isautomatically configured within the EISA slot-specific I/O addressrange, as would be appreciated by one of ordinary skill in the computerfield. Additionally, EISA product ID I/O registers are enabled so thatthe host adapter 18' appears to be an EISA board to the host CPU 10.

In an ISA system, the I/O base address is set up by the host CPU 10 ashereinafter described and may be any address in the host CPU 10's I/Oaddress space which does not conflict with another device. Afterpower-up or reset, the host CPU 10 writes to address 03FFh. The host CPU10 writes a sequence of bytes to this register which forms a hostadapter configuration command. This configuration command is a four-bytecommand which has the following format:

                  TABLE 1                                                         ______________________________________                                        BYTE     DESCRIPTION                                                          ______________________________________                                        F0h      Command Byte                                                         00       Logical Board Number (1-4), Physical ID (3,                                   5, 6, 7)                                                             xx       low-order bits of I/O base address to set                            yy       high-order bits of I/O base address to set                           ______________________________________                                    

These command bytes are read by the host adapter CPU 40 which in turnprograms the bus master control logic 56 to set the requested I/O baseaddress. Once the configuration command has been issued by the host CPU10, the host CPU 10 may verify the presence of the host adapter 18' atthe specified base address by issuing further commands. The host CPU 10may repeat the configuration command sequence for up to four boards andby doing so, determine the number of host adapters such as 18' that areinstalled in the system.

COMMUNICATION BETWEEN THE HOST CPU 10 AND THE HOST ADAPTER 18'

As previously noted, communication between the host CPU 10 and the hostadapter 18' in accordance with the present invention is supported by themail registers 43 which are provided by the host adapter 18' and mappedinto the I/O address space of the host CPU 10. The mail registers 43 aredefined in Table 2 as follows:

                  TABLE 2                                                         ______________________________________                                        MAIL REGISTERS 43                                                                                     REGISTER                                              HOST I/O ADDRESS                                                                            MODE      DEFINITION                                            ______________________________________                                        I/O base address                                                                            read/write                                                                              R0, 16-bit Mailbox                                                            Register                                              I/O base address + 2                                                                        read/write                                                                              R1, 16-bit Mailbox                                                            Register                                              I/O base address + 4                                                                        read/write                                                                              R2, 16-bit Mailbox                                                            Register                                              I/O base address + 6                                                                        read/write                                                                              R3, 16-bit Mailbox                                                            Register                                              I/O base address + 7                                                                        read only R4, 8-bit Mail Status                                                         Register                                              I/O base address + 8                                                                        write only                                                                              R5, 8-bit Mail Command                                                        Register                                              ______________________________________                                    

The mail registers 43 provide the interface between the host CPU 10 andthe host adapter 18'. Each of the registers in the mail registers 43 ismapped into the host CPU 10's I/O address space according to Table 2.Referring to Table 2, the mail registers 43 include four 16-bit"mailbox" registers, an 8-bit "mail status" register, and an 8-bit "mailcommand" register. The mailbox registers are bidirectional,general-purpose data pathways between the host CPU 10 and the hostadapter CPU 40.

The mail status register R4 holds eight bits which enable implementationof a software communication protocol between the host CPU 10 and thehost adapter 18'. The bits of the mail status register R4 are defined inTable 3 as follows:

                  TABLE 3                                                         ______________________________________                                        MAIL STATUS REGISTER R4                                                       BIT       THE HOST CPU 10 INTERPRETATION                                      ______________________________________                                        0         Mailbox register R0 input full                                      1         Mailbox register R1 input full                                      2         Mailbox register R2 input full                                      3         Mailbox register R3 input full                                      4         Mailbox register R0 output full                                     5         Mailbox register R1 output full                                     6         Mailbox register R2 output full                                     7         Mailbox register R3 output full                                     ______________________________________                                    

The status bits in the mailbox status register R4 allow for theimplementation of a software protocol for communication of informationbetween the host CPU 10 and the host adapter CPU 40. For example, whenbit 0 of the mail status register R4 is set to "1" this indicates thatunread data from the host adapter CPU 40 exists in the mailbox registerR0; when this data is read from mailbox register R0 by the host CPU 10,bit 0 of mail status register R4 is cleared to "0". When bit 4 of themail status register R4 is set to "1" this indicates that unread datafrom the host CPU 10 exists in mailbox register R0; when this data isread from the mailbox register R0 by the host adapter CPU 40, bit 4 ofthe mail status register R4 is cleared to "0". This provides anindication that the mailbox register R0 may be written to withoutoverwriting previously written data.

The hardware of the present invention supports the softwarecommunication protocol between the host CPU 10 and the host adapter 18'by updating the bits of the mail status register R4. In particular, whenthe host CPU 10 reads information from one of the mailbox registersR0-R3, host adapter logic (not shown) clears the respective input statusbit 0-3 of the mail status register R4. Conversely, when the hostadapter CPU 40 reads information from one of the mailbox registersR0-R3, host adapter logic clears the respective output status bit 4-7 inthe mail status register R4.

The mail command register R5 provides a direct pathway for the host CPU10 to elicit the attention of the host adapter CPU 40, or to clear theinterrupt state from the host adapter 18'. The bits of mail commandregister R5 are defined in Table 4 as follows:

                  TABLE 4                                                         ______________________________________                                        MAIL COMMAND REGISTER R5                                                      BIT  EFFECT WHEN WRITTEN BY THE HOST CPU 10                                   ______________________________________                                        0    Resets all logic in host the adapter 18'                                 1    Interrupts the host adapter CPU 40                                       2-7  not used                                                                 ______________________________________                                    

All subsystems interfaces to the host adapter CPU 40 are memory mappedin a conventional manner. Such mapping allows the broadest selection ofaddressing modes and simplifies implementation of high-level languagecontrol programs to be executed by the host adapter CPU 40.

The RAM 42 is, in the preferred embodiment, a 64 kbyte, one wait statememory module. The placement of the RAM 42 in the memory map of the hostadapter CPU 40 is determined by the fact that the host adapter CPU 40uses the lower 1024 bytes of its address space to contain interruptvector tables. Eight to twelve kbytes of the RAM 42 are used by thecontrol program and data structures. Thirty-six kbytes of the RAM 42 arereserved for device input and output buffers. The remainder of the RAM42 may be used for dynamic, high-demand burst input and output buffers.

As noted above, the microprocessor support logic 41, the bus mastercontrol logic 56, the HSRTs 48, 50, 52, and 54, the mail registers 43and the IRQ logic 49 are implemented in an ASIC. The ASIC manages theI/O communications registers between the host CPU 10 and the hostadapter 18'. There are six mail registers in the memory address space ofthe host adapter CPU 40 which correspond to the six mail registers 43which are mapped in the host I/O address space. Mailbox registers R0'through R3' are 16-bit bidirectional data pathways between the hostadapter CPU 40 and the host CPU 10. There are actually twounidirectional pathways (in opposite directions) associated with each ofthe general-purpose mailbox registers. A mail status register R4'provides flow control and status information for the first four mailboxregisters R0' through R3'. The mail status register R4' is similar tothe mail status register R4 for the host, although the mapping betweeninput and output is reversed since the direction is relative to the hostadapter CPU 40 instead of the host CPU 10. The definitions of the eightbits of mail status register R4' is set forth in Table 5 as follows:

                  TABLE 5                                                         ______________________________________                                        MAIL STATUS REGISTER R4'                                                      BIT     HOST ADAPTER CPU 40 INTERPRETATION                                    ______________________________________                                        0       Mailbox register R0 output full                                       1       Mailbox register R1 output full                                       2       Mailbox register R2 output full                                       3       Mailbox register R3 output full                                       4       Mailbox register R0 input full                                        5       Mailbox register R1 input full                                        6       Mailbox register R2 input full                                        7       Mailbox register R3 input full                                        ______________________________________                                    

Similar to the mail status register R4, the contents of register R4' maybe used to implement a software-based flow control and command passingprotocol. No hardware enforcement of such a protocol is provided toprevent overwriting of data in mailbox registers R0' through R3'. A mailcommand register R5' provides the capability for the host adapter CPU 40to generate an interrupt to the host CPU 10 or to clear interrupt statefrom the host. The mail command register R5' is bit-mapped as set forthin Table 6 as follows:

                  TABLE 6                                                         ______________________________________                                        MAIL COMMAND REGISTER R5'                                                          EFFECT                                                                   BIT  WHEN WRITTEN BY THE HOST ADAPTER CPU 40                                  ______________________________________                                        0    Causes an interrupt to the host CPU 10                                   1    Indicates end-of-interrupt processing by the                                  host adapter CPU 40                                                      2-7  not used                                                                 ______________________________________                                    

A number of registers are associated with the programming of bus mastertransfers to or from memory associated with the host CPU 10. All busmaster transfers are specified by the host adapter CPU 40. Fourparameters must be specified for a transfer operation: The host CPU 10address, the host adapter CPU 40 address, transfer byte count, andtransfer direction. Both the host CPU 10 and the host adapter CPU 40addresses must be specified as physical (RAM) addresses. Although thebest performance is obtained on transfers from even addresses to evenaddresses, the bus master control logic 56 supports transfers to andfrom even and odd addresses.

A maximum of 64 kbytes may be transferred in one bus master request.Since only a limited number of words of data may be transferred eachtime the bus master control logic 56 is granted control of the host bus20, a single transfer request submitted by the host adapter CPU 40 willresult in one or more requests for the host bus 20 by the bus mastercontrol logic 56. This subdivision of block transfers and all otheraspects of host bus arbitration are transparent to the host adapter CPU40.

Once the transfer byte count, the host CPU 10 and the host adapter 18'memory addresses have been written to the proper registers in the busmaster control logic 56, a write to a transfer control registerinitiates the transfer. The transfer control register is bit-mapped asset forth in Table 7 as follows:

                  TABLE 7                                                         ______________________________________                                        TRANSFER CONTROL REGISTER                                                     BIT       INTERPRETATION                                                      ______________________________________                                        0         Transfer direction:                                                           (0 means from the host CPU 10 to the host                                     adapter 18'                                                                   1 means from the host adapter 18' to the                                      host CPU 10)                                                        1-6       not used                                                            7         Reset/Abort Transfer                                                ______________________________________                                    

Setting the reset bit 7 of the transfer control register terminates anytransfers in progress and resets the bus master control logic 56completely.

The transfer specification registers are internally buffered by the busmaster control logic 56 to allow loading of new transfer specificationparameters while a transfer is in progress. A bus master status registeris bit-mapped as set forth in Table 8 below to indicate the condition ofthe internal, transfer-in-progress parameter registers and the inputtransfer specification registers:

                  TABLE 8                                                         ______________________________________                                        BUS MASTER STATUS REGISTER                                                    BIT         INTERPRETATION                                                    ______________________________________                                        0           Transfer in progress                                              1           Transfer specification registers full                             2           Parent-Child Mode                                                 3-7         not used                                                          ______________________________________                                    

By polling the bus master status register, the host adapter CPU 40 maydetermine when a requested transfer has completed. A transfer queuingmechanism may be implemented in software. A current transfer remainingcount register may also be read by the host adapter CPU 40.

As noted above, the HSRT's 48, 50, 52 and 54 are implemented in an ASICon the host adapter 18'. Each of the HSRT's 48, 50, 52 and 54 arecapable of communicating with a similar HSRT located on each dataconcentrator, such as the data concentrator 28. The implementation andmethod of communication between the host adapter 18' and the dataconcentrator 28 through the HSRT's will be discussed thoroughly below inthe section describing the data concentrator 28.

The complete memory map of the host adapter CPU 40 is set forth in Table10:

                  TABLE 10                                                        ______________________________________                                        HOST ADAPTER CPU 40 MEMORY MAP                                                ADDRESS LENGTH              DESCRIPTION                                       (HEX)   (BYTES)   MODE      OF ITEM                                           ______________________________________                                        RAM 42                                                                        00000   64k       read/write                                                                              RAM (32k words)                                   ASIC                                                                          10000   2         read/write                                                                              Mailbox register R0                               10002   2         read/write                                                                              Mailbox register R1                               10004   2         read/write                                                                              Mailbox register R2                               10006   2         read/write                                                                              Mailbox register R3                               10008   1         read only Mail Status register                                                          R4                                                10009   1         write only                                                                              Mail Command register                                                         R5                                                1000A   2         read/write                                                                              Programmable timer                                                            count register                                    1000C   2         read only Programmable host I/O                                                         base address                                      1000E   2         write only                                                                              Programmable initial                                                          I/O address                                       10010   1         write only                                                                              Interrupt vector                                                              number for octal UART                                                         46--Receive Buffer                                                            full                                              10011   1         write only                                                                              Interrupt vector                                                              number for octal UART                                                         46--Transmit Buffer                                                           empty                                             10012   1         write only                                                                              Interrupt vector                                                              number for octal UART                                                         46--Modem status                                                              change                                            10013   1         write only                                                                              Interrupt vector                                                              number for timer tick                             10014   1         write only                                                                              Interrupt vector host                                                         to host adapter CPU 40                            10015   1         write only                                                                              Board ID (switch                                                              position)                                                           bit map                                                                       7         1 = EISA System                                                               Detected                                                            6         1 = EISA Override                                                             Enabled                                                             5-3       0 = Bus Master Control                                                        logic 56 Version 0                                                  2-0       Board Id bits:                                                                011 = board 1                                                                 101 = board 2                                                                 111 = board 3                                                                 110 = board 4                                     10016   10        --        Test Register for                                                             hardware                                          10018   1         read/write                                                                              Bus Master Timer                                  1001A   1         read/write                                                                              EISA revision byte                                BUS MASTER 56                                                                 10020   2         read/write                                                                              Host Address A2 (lower                                                        16 bits)                                          10022   1         write only                                                                              Host Address A0 (upper                                                        8 bits)                                           10023   1         --        not used                                          10024   2         read/write                                                                              host adapter CPU 40                                                           address A2 (lower 16                                                          bits)                                             10026   1         --        Reserved                                          10027   1         --        not used                                          10028   2         write only                                                                              Transfer Count                                    1002A   1         write only                                                                              Transfer Control                                                              register                                          1002B   1         read only Bus Master Status                                                             register                                          1002C   2         read only Transfer Remaining                                                            Count register                                    1002E   1         read/write                                                                              Parent-child                                                                  Designation register                                                bit map                                                                       0         1 = Child Board                                                               (default)                                                                     0 = Parent Board                                                    1         1 = EISA override                                                             enabled                                                             7-2       Reserved                                          HSRTs 48, 50, 52 and 54                                                       10040   2         read/write                                                                              HSRT 48 data                                      10042   2         read/write                                                                              HSRT 50 data                                      10044   2         read/write                                                                              HSRT 52 data                                      10046   2         read/write                                                                              HSRT 54 data                                      10048   1         read only HSRT 48 status                                    10049   1         read only HSRT 50 status                                    1004A   1         read only HSRT 52 status                                    1004B   1         read only HSRT 54 status                                    1004C   20        not used                                                    10060   32        not used                                                    DISCRETE                                                                      10080   1         write only                                                                              DMA and interrupt                                                             program register                                                    bit map                                                                       7         1 = Select Host DMA                                                           CH7                                                                 6         1 = Select Host DMA                                                           CH6                                                                 5         1 = Select Host DMA                                                           CH5                                                                 4         1 = Select Host DMA                                                           CH3                                                                 3         1 = Select Host IRQ 3                                               2         1 = Select Host IRQ 5                                               1         1 = Select Host IRQ 10                                              0         1 = Select Host IRQ 11                            10081   127       --        not used                                          10100   1         read only Interboard                                                                    communications 47                                 10101   127       --        not used                                          10180   1         write only                                                                              Interboard                                                                    communications 47                                 10181   127       --        not used                                          10200   1         write only                                                                              Data concentrator 28                                                          mode enable                                                         bit map                                                                       7-4       Not used                                                            3         1 = Enable HSRT 48                                                            Port S0                                                             2         1 = Enable HSRT 50                                                            Prot S1                                                             1         1 = Enable HSRT 52                                                            Port S2                                                             0         1 = Enable HSRT 54                                                            Port S3                                           10210   127       --        not used                                          10280   1         write only                                                                              Data terminal ready                               10281   127       --        not used                                          10300   128       read/write                                                                              Octal UART 46                                     10380   973952    --        not used                                          ROM 44                                                                        FE000   8k        read only Boot strap entry point                            FFFF0   5         read only Reset entry point,                                                            jump to FE000h                                    FFFF5   11        read only ROM 44 version and                                                            date                                              ______________________________________                                    

THE HOST ADAPTER 18' FUNCTIONS

The code present in the ROM 44 performs several types of functions. Thecode in the ROM 44 includes the reset entry point and adapterinitialization code, as well as a communications and terminal I/Ocontrol program which interfaces to host device drivers executed by thehost CPU 10. Diagnostic routines are also included in the ROM 44 and maybe invoked to test various subsystems of the host adapter 18' from astand-alone diagnostic program or to trouble-shoot the host adapter 18'during normal operation.

The first function performed by the host adapter CPU 40 following resetis a consistency check of subsystems of the host adapter 18'. The hostadapter CPU 40 computes a checksum of the contents of the ROM 44 andverifies the checksum with a preset value stored in the ROM 44. The hostadapter CPU 40 sets the host I/O initialization address and checks busmaster register values for expected reset values.

Next, the host adapter CPU 40 performs write and read verify tests onthe RAM 42, copies portions of code stored in the ROM 44 into the RAM 42for faster execution, and sets up a service routine address for the hostcommand interrupt. An integrity check completion status is then writtento the mailbox register R0.

AUTOMATIC DATA CONCENTRATOR 28 DETECTION

As part of the initialization process and periodically thereafter, thehost adapter CPU 40 uses the procedure described below to determinewhich, if any, of the ports S0-S3 on the adapter board fan-out device 64are coupled to functioning data concentrators, such as the dataconcentrator 28 in FIG. 3. Four ports SO through S3 are switchablebetween RS-232 asynchronous mode for connection to serial devices andRS-422 synchronous mode for connection to data concentrators. Theswitching capability is performed by the data concentrator select logic45 and the relays 68a-68d. Ports S0 through S3 power-up in RS-232 mode,and remain so configured unless a data concentrator is positivelydetected.

In order make its presence known to host adapter 18' the dataconcentrator 28 of FIG. 4 will, upon power-up and periodicallythereafter, send an ID response packet to the host adapter 18'. Sinceflow control is embedded in the synchronous link between the dataconcentrator 28 and the host adapter 18', data to be sent by themicrocontroller 74 (FIG. 4) of the data concentrator 28 will be placedin the transmit register of the HSRT 72. The data will remain in theHSRT 72, pending transmission, until the RS-422 receivers 60 (see FIG.3) are enabled.

On power-up or reset, the host adapter CPU 40 enables the RS-422receivers 60 for a short period of time. If data concentrators, such asthe data concentrator 28 of FIG. 4, are connected and powered up, thehost adapter CPU 40 will receive data stored in the HSRT 72 and therebydetect the presence of the data concentrator 28 in a passive manner,without itself transmitting data which could result in "garbage" databeing received by a terminal, printer, or other serial device which maybe connected to ports SO through S3. Any of the configurable ports S0through S3 which may not be determined to be connected to a dataconcentrator will be returned to RS-232 mode. Periodically, duringnormal operation and during open processing on a port which may beconnected to a data concentrator, the host adapter CPU 40 willtemporarily enable the RS-422 receivers 60 in receive mode and check forvalid data reception.

In the presently disclosed embodiment, the preferred method of power-upis that all the data concentrators be connected and powered-up prior tobooting the host system. It is also preferred that reconfiguration ofconnections to the data concentrators not be undertaken during normaloperation of the host CPU 10.

HOST CPU 10 TO HOST ADAPTER 18' COMMAND PROTOCOL

The host CPU 10 can receive information and send commands to the hostadapter 18' through a command protocol in accordance with the preferredembodiment of the invention. This communication link allows the host toreceive version, status and configuration information as well as set DMAchannels and IRQ's, initialize data structures, and configure the highspeed channels and the remote devices coupled through the host adapter18'.

For example, the device drivers of the host may not directly access theremote devices, such as the devices 12, 14 and 16, but must sendcommands to the host adapter 18' so that the host adapter 18' canconfigure the remote devices 12, 14 and 16. The device drivers must workin conjunction with a software configuration and database managementutility as well as with the host adapter 18' to properly configure eachremote device coupled through the host adapter 18'. The managementutility configures the operating system of the host which builds andmaintains a cross-reference data file comprising the unique identity andconfiguration of each remote device. The data file is maintained on thehost system, such as on a hard disk of the host computer. The hostoperating system uses the cross-reference data file to control thedevice drivers to establish the configuration of the remote devices. Thedevice drivers send configuration information through commands pursuantto the command protocol, and the host adapter 18' configures each deviceeither directly, or indirectly through the data concentrators pursuantto the commands sent by the device drivers. The host adapter 18' alsoassigns unique identifiers to each remote device, and these identifiersare passed back to the host and are kept in the cross-reference datafile.

In accordance with the presently disclosed embodiment of the invention,commands are passed from the host CPU 10 to the host adapter 18' via themailbox register R0. The mailbox registers R1, R2, and R3 are used forpassing command-specific parameters. Commands are specified by the lowereight bits of register R0. For some commands, the upper eight bits ofregister R0 contain additional command parameters or modifiers.

Most commands are handled synchronously, such that the host adapter CPU18' completes all processing of a command before beginning to process anext command. FIG. 5 illustrates in flowchart form the sequence ofevents which occur during the processing of a host command. Node 100 inthe flowchart of FIG. 5 represents the point where a user processexecuted by the host CPU 10 issues a system call which requires acommand to be issued to the host adapter 18' by the host device driver.Control of the host CPU 10 is subsequently turned over to the devicedriver.

The device driver first determines whether a command is currently beingprocessed by the host adapter 18' as represented in decision node 102 ofFIG. 5. If a command is not already in progress, the host device driverwaits for the mailbox register R0 to be available for writing; this stepis represented by node 104 in FIG. 5. If the device driver is sequencingcommands to the host adapter 18, the mailbox register R0 should beavailable immediately. If the mailbox register R0 is not availableimmediately, an error or exceptional condition in the driver or the hostadapter 18' has occurred, and an error handling sequence is executed asrepresented by node 105 of FIG. 5. If the mailbox register R0 isavailable, the device driver writes the command and associatedparameters into the mailbox registers R0-R3; this stage is representedby node 106.

Returning to the node 102 of FIG. 5, if a command is currently beingprocessed by the host adapter 18', the user process which issued thecommand is suspended, as represented by node 108 in FIG. 5. As shall behereinafter described, when a command is completed by the host adapter18' and the results received by the device driver in the mailboxregisters R0-R3, the device driver then "wakes up" a process, if any,that is waiting for command completion at the node 108 of the flowdiagram of FIG. 5. When the process is awakened, the device driverwrites the command and associated parameters, as shown in node 106.

After the command and parameters have been written into the mailboxregisters R0-R3, the device driver interrupts the host adapter CPU 40 bywriting to the mail command register R5; this is represented by node 110of FIG. 5. In the node 110, the device driver sets bit 1 in the mailcommand register R5 which alerts the host adapter 18' that it shouldinterrupt the host CPU 40 after the command written to the mailboxregister R0 in the node 106 has been processed.

Next, the device driver suspends the process which issued the commandpending command completion, as shown in node 112 of FIG. 5. Nodes 114,116, and 118 of FIG. 5 represent processing steps taken by the hostadapter CPU 40. First, in the node 114, the host adapter CPU 40 executesthe command identified in the mailbox register R0, and the commandresponse is written to the mailbox registers as shown in the node 116.Then the host adapter CPU 40 causes an interrupt to the host CPU 10 bywriting to the mail command register R5' as represented by the node 118

When the host CPU 10 receives the interrupt, the device driver wakes upthe process suspended in the node 112, as represented by node 120. Whenthis process resumes, it will detect that the command has beencompleted. If another process is awaiting the completion of commandprocessing as suspended in the node 108, the device driver now wakes upthat process, as represented by node 122, and the awakened processshould find the mailbox register R0 now available for writing.

When configured to operate without host interrupts, the processing doneby the interrupt service routine in the sequence of events illustratedin FIG. 5 would be performed by a periodic polling routine. This wouldhave the effect of limiting the number of commands processed by theadapter in a given period of time according to the frequency of thepolling routine.

A few commands require an external event for completion. The sequencefor handling such commands would be similar to that described withreference to FIG. 5 above, except that when the host CPU 10 isinterrupted following the submission of an asynchronous command, theresponse from the host adapter 18' indicates that the command is inprogress. Subsequently, on actual command completion, the host CPU 10will be interrupted again. Asynchronous command parameters include acommand reference handle which is passed back on command completion. Thehost device uses this reference handle to identify which asynchronouscommand has completed and resumes the appropriate process.

The command set for the host adapter 18' is set forth in Table 11 below:

                  TABLE 11                                                        ______________________________________                                        HOST ADAPTER 18' COMMAND SET                                                  COMMAND VALUE FUNCTION                                                        ______________________________________                                        0000          Get host adapter 18' version                                                  information                                                     0001          Get host adapter 18' extended                                                 information                                                     0002          Set DMA channel and IRQ                                         0003          Initialize transfer data structures                             0004          Yield transfer data structures                                  0005          Get host adapter 18' status                                     0006          Get host adapter 18'/data                                                     concentrator 28 port configuration                              nn07          Open tty port                                                   nn08          Close tty port                                                  nn09          Set tty port mode                                               nn0A          Set tty port extended mode                                      nn0B          Transmit break                                                  nn0C          Flush tty port input                                            nn0D          Flush tty port output                                           nn0E          Flush tty port input and output                                 nn0F          Wait for tty port output to drain                               nn10          Wait for tty port modem status change                           nn11          Cancel tty port wait command                                    nn12          Get tty port status                                             0013          Open direct high speed channel                                  0014          Close direct high speed channel                                 0015          Reset direct high speed channel                                 0016          Get direct high speed channel status                            0017          Shutdown all ports and channels                                 nn18          Execute diagnostic routine                                      0019          Peek host adapter RAM 42                                        001A          Poke host adapter RAM 42                                        001B          Load new executable image                                       ______________________________________                                    

Many commands in Table 11 above, such as those which change or return aport status, require very little processing by the host adapter 18'.During command processing by the host adapter 18', polling of the HSRT's48, 50, 52, and 54 is suspended. In general, the number and frequency ofcommands issued in the host adapter 18' by the host device driver isnegligible compared to the amount of transmitted and received data. Thistransmitted and received data is not passed via the mailbox commandregisters, but is directly bus mastered by the host adapter 18' to andfrom the host CPU 10 memory as will be described in the section entitled"Device I/O Data Transfer Protocol", below.

HOST ADAPTER 18' CONFIGURATION AND STATUS COMMANDS

Several of the commands in Table 11 allow the host adapter 18' to beconfigured and provide version information to the host device driver.The commands which return the host adapter 18' information are intendedto insulate the host device driver from changes to the host adapter 18'hardware or firmware in future versions. Radical changes to the hostadapter 18' may not allow the host device driver to be completelyinsulated, but at least those commands will allow the device driver todetect and perhaps adapt to changes in the host adapter 18'.

For example, a future version of the host adapter 18' might support alarger number of ports. This would have minimal impact on a driver whichqueried the host adapter 18' to determine the maximum port number for aparticular adapter. However, a later version of the host adapter 18'might include support for additional commands. If such an adaptermaintained backwards compatibility, a host device driver written for anearlier version would function properly with the new adapter. Newerversions of the driver could take advantage of the additional commandssince it could register their availability through a request to the hostadapter 18'.

The following is a description of each of the host adapter 18'configuration and status commands supported in the presently disclosedembodiment of the invention:

Get Host Adapter 18' Version Information Command

This command allows the host device driver to obtain firmware and thehost adapter 18' hardware version information. This may allow the devicedriver to support future versions of the host adapter 18' which provideadditional functionality or features. This command has one parameter:

    ______________________________________                                        R0      Get host adapter version information command                          ______________________________________                                    

The command returns the following information:

    ______________________________________                                        R0       command completion status                                            R1       host adapter hardware revision                                       R2       host adapter firmware revision                                       R3       host adapter RAM and number of connectors                            ______________________________________                                    

Get Host Adapter 18' Extended Information Command

This command allows the host device driver to obtain more informationabout the host adapter 18'. This may allow the device driver to supportfuture versions of the adapter which provide additional functionality orfeatures. This command has one parameter:

    ______________________________________                                        R0        Get host adapter 18' extended information                                     command                                                             ______________________________________                                    

The command returns the following information:

    ______________________________________                                        R0          command completion status                                         R1          DMA channels supported bit-map                                    R2          IRQ levels supported                                              R3          current configuration:                                                        DMA channel (high byte)                                                       IRQ level (low byte)                                              ______________________________________                                    

Set DMA Channel and IRQ Command

This command configures the interrupt request (IRQ) and DMA channel tobe used by the host adapter 18'. The command requires three parameters:

    ______________________________________                                        R0            Set DMA channel and IRQ                                         R1            DMA select (3, 5, 6, 7)                                         R2            IRQ select (-1, 3, 5, 10, 11)                                   ______________________________________                                    

The host adapter 18' must have a DMA channel reserved for its use;however, it can operate in a polled environment without and does notrequire a host interrupt. If the IRQ select value is (-1), the hostadapter 18' will never interrupt the host CPU 10. If either the DMA orIRQ select values are invalid, an error code is returned to the host CPU10. Otherwise a successful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Initialize Transfer Data Structure Command

The host device driver allocates several data structures in host RAM foruse by the host adapter 18' firmware. The significance of these datastructures shall be hereinafter described. This command is used to passthe address of the data structure to the host adapter 18'. The commandhas three parameters:

    ______________________________________                                        R0         Initialize transfer data structures command                        R1, R2     Physical host RAM address of transfer                                         control structure                                                  R3         Size, in bytes, of the transfer control data                                  structure                                                          ______________________________________                                    

If the transfer control data structure size or contents are incompatiblewith the firmware revision of the host adapter 18' an error code isreturned to the host CPU 10. Otherwise, a successful completion statusis returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Yield Transfer Data Structure Indication

Periodically, the host adapter 18' yields use of the transfer datastructure to the host device driver. The host CPU 10 must inform thehost adapter 18' when it is through processing these data structures.This indication has one parameter:

    ______________________________________                                        R0         Yield transfer data structures indication                          ______________________________________                                    

This is not a command to the host adapter 18' but rather an indication.As such, no completion status is generated by the host adapter 18'.

Get Host adapter 18' Status Command

This command returns high-level host adapter 18' status information.This command has one parameter:

    ______________________________________                                        R0         Get host adapter status command                                    ______________________________________                                    

The following information is returned:

    ______________________________________                                        R0             command completion status                                      R1, R2, R3     host adapter 18' status                                        ______________________________________                                    

Get Host Adapter 18'/Data Concentrator 28 Configuration Command

This command returns configuration information regarding theasynchronous port and high-speed channel connectors. The command has twoparameters:

    ______________________________________                                        R0        Get host adapter 18'/data concentrator 28                                     configuration command                                               R1        connector number                                                    ______________________________________                                    

The following information is returned:

    ______________________________________                                        R0      command completion status                                             R1      connection type/status bit map:                                       Bit         Definition                                                        0001h       RS-232 asynchronous capable                                                   connector                                                         0002h       High-speed capable connector                                      0004h       RS-232 asynchronous mode enabled                                  0008h       High-speed mode enabled                                           0010h       Data concentrator 28 attached                                     FFE0h       not used                                                          R2      number of RS-232 asynchronous ports supported                                 (directly or through a data concentrator)                             ______________________________________                                    

TTY PORT COMMANDS

The following is a description of the commands which allow the hostdevice driver to control local and remote RS-232 ports. The open commandestablishes a connection between I/O buffers and data structures in thedevice driver and corresponding data structures on the host adapter 18'.The close command breaks this connection and disables data transmissionor reception on a port. Other TTY port commands support host devicedriver requirements. For all port commands, the upper eight bits of themailbox register R0 contains the port identification number.

Open TTY Port

This command changes the state of a specified port from closed to open.The command has two parameters:

    ______________________________________                                        R0             command completion status                                      R1, R2         adapter reference identifier                                   ______________________________________                                    

The host reference identifier is used in subsequent communications withthe host device driver. In operation, this is intended to be a pointerto a data structure in the host RAM which is related to a specifiedport. Its value and usage by the host CPU 10 are transparent to the hostadapter 18'. Likewise the adapter reference handle is used by the hostdevice driver in subsequent communications with the host adapter 18'.

This command is called only for the first open of a TTY device. Themapping between the file system TTY device node number and the portnumber on the host adapter 18' is maintained by the host device driver.

Close TTY Port Command

This command changes the state of a specified port from open to closed.In the closed state, a port can neither send nor receive data and modemcontrol lines are placed in inhibited state. The port operatingparameters are reset to default values. The command has one parameter:

    ______________________________________                                        R0         Port number/Close TTY port command                                 ______________________________________                                    

The port number must be between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is already closed, an error code is returned to the host; otherwisea successful completion status is returned.

Set TTY Port Mode Command

This command allows the host CPU 10 to set up a number of TTY portoperating parameters in a format which is compatible with terminal datastructures, such as UNIX Standard TERMIO data structures. User processesset TTY input, output and control modes for a TTY. The bit-mappedcontrol values for these modes may be passed directly to the hostadapter 18' for processing, although some modes are not applicable forhandling on the host adapter 18'.

Although this command may be optimized for a UNIX device driver, itprovides a concise method for passing configuration information to thehost adapter 18' which may be useful in other operating environments. Asrequired, other commands to set TTY line parameters and operating modesmay be added to the command set supported by the presently disclosedembodiment of the invention. One such set of commands might provide alevel 8250 UART command register compatibility.

The "Set TTY Port Mode" command has four parameters:

    ______________________________________                                        R0       Port number/Set TTY Port Mode Command                                R1       Input mode descriptor                                                R2       Output mode descriptor                                               R3       Control mode descriptor                                              ______________________________________                                    

When a port is opened, its mode is initialized to a system default modesetting. This command may be used to specify the default open port modeby specifying the port number as -1 (0×FF).

The port number must be between zero and the maximum number of portssupported by the host adapter 18', or must be the special value to setthe default input mode setting. If the port number is invalid or theport is closed, an error code is returned to the host. Otherwise, asuccessful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

TTY Port Input Mode Descriptor

This descriptor configures received character processing for thespecified port and is bit-mapped as follows:

    ______________________________________                                        Bit      Definition                                                           ______________________________________                                        0001     Ignore break condition                                               0002     Interrupt on break reception                                         0004     Ignore characters received with parity errors                        0008     Mark parity errors                                                   0010     Enable input parity checking                                         0020     Strip characters to seven bits                                       0040     Map NL to CR on input                                                0080     Ignore CR on input                                                   0100     Map CR to NL on input                                                0200     Map uppercase to lowercase on input                                  0400     Enable start/stop output control                                     0800     Enable any character to restart output                               1000     Enable start/stop input control                                      ______________________________________                                    

TTY Port Output Mode Descriptor

The output mode descriptor configures the transmitted characterprocessing for the specified port and is bit-mapped as follows:

    ______________________________________                                        Bit        Definition                                                         ______________________________________                                        0001       Post-process output                                                0002       Map lowercase to uppercase on output                               0004       Map NL to CR-NL on output                                          0008       Map CR to NL on output                                             0010       No CR output at column 0                                           0020       NL performs CR function                                            0040       Use fill characters for delay                                      0080       Use DEL for fill, else NUL                                         0100       Select newline delay                                               0600       Select carriage return delay                                       1800       Select horizontal tab delay                                        2000       Select backspace delay                                             4000       Select vertical tab delay                                          8000       Select form feed delay                                             ______________________________________                                    

TTY Port Mode Descriptor

This descriptor configures the hardware control for the specified portand is bit-mapped as follows:

    ______________________________________                                        Bit         Definition                                                        ______________________________________                                        000F        Select baud rate                                                            0000 Hang up                                                                  0001 50                                                                       0002 75                                                                       0003 110                                                                      0004 134.5                                                                    0005 150                                                                      0006 200                                                                      0007 300                                                                      0008 600                                                                      0009 1200                                                                     000A 1800                                                                     000B 2400                                                                     000C 4800                                                                     000D 9600                                                                     000E 19200                                                                    000F 38400                                                          0030        Select character size                                                       0000 5 bits                                                                   0010 6 bits                                                                   0020 7 bits                                                                   0030 8 bits                                                         0040        Select two stop bits, else one                                    0080        Enable receiver                                                   00100       Enable parity                                                     0200        Select odd parity, else even                                      0400        Hang up on last close                                             0800        Select local line mode, else dial-up                              1000        Block layer output                                                2000        Enable CTS protocol                                               4000        Enable RTS signalling                                             ______________________________________                                    

Set TTY port Extended Mode Command

This command sets up the specified port for other operating modes suchas internal loop-back and alternative hardware flow signalling protocolswhich are not configurable through the "Set TTY Port Mode" command. Thecommand has two or three parameters:

    ______________________________________                                        R0     Port number/Set TTY port extended mode command                         R1     Mode to Set                                                            0        Internal loop-back                                                   1        Alternative flow control as specified                                         by R2                                                                2        Suspend transmit and receive                                         3        Resume transmit and receive                                          R2     Alternative flow control to set (mode 1)                               0        None                                                                 1        DSR flow control                                                     ______________________________________                                    

The internal loop-back mode enables a diagnostic mode of the UART 46 forthe specified port which echoes transmitted data back as received data.The alternative flow controls provide support for various flow controlsignalling conventions not supported by the standard TTY mode settings.

The suspend and resume command modes allow direct flow controlmanipulation under process control. On receipt of a suspend command thehost adapter 18' will stop transmitting and issue a request to thedevice to stop transmitting. The port will remain in this state until aresume command is received or the port is closed. A small amount of databuffered in the data concentrator 28 for the specified port may betransmitted or received even after the suspend command has beenprocessed.

The port number must be between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is closed, an error code is returned to the host CPU 10. Otherwisea successful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Transmit Break Command

This command causes a break to be transmitted on the specified port. Thecommand has one parameter:

    ______________________________________                                        R0         Port number/Transmit break command                                 ______________________________________                                    

The port number must be between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is closed, an error code is returned to the host CPU 10. Otherwise,a successful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Flush TTY Port Input Command

This command disposes of any received data for the specified port whichremains buffered on the host adapter 18' and the data concentrator 28.The command has one parameter:

    ______________________________________                                        R0       Port number/Flush TTY port input command                             ______________________________________                                    

The port number must be between zero and the maximum number of portsupport by host adapter 18'. If the port number is invalid or the portis closed, an error code is returned. Otherwise a successful completionstatus is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Flush TTY Port Output Command

This command disposes of any transmitted data for the specified portwhich remains buffered on the host adapter 18' and the data concentrator28. The command has one parameter:

    ______________________________________                                        R0      Port number/Flush TTY Port Output Command                             ______________________________________                                    

The port number must be between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is closed, an error code is returned to the host. Otherwise asuccessful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Flush TTY Port Input and Output Command

This command disposes of any received and transmitted data for thespecified port which remains buffered on the host adapter 18' and thedata concentrator 28. The command has one parameter:

    ______________________________________                                        R0      Port Number/Flush TTY port input and output                                   command                                                               ______________________________________                                    

The port number must between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is closed, an error code is returned to the host. Otherwise asuccessful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Wait for TTY Port Output to Drain Command

This command notifies the host CPU 10 when the specified TTY port outputbuffer is empty on the host adapter 18' and the data concentrator 28.The command has two parameters:

    ______________________________________                                        R0         Port number/Wait for TTY port output to                                       drain command                                                      R1, R2     host command reference                                             ______________________________________                                    

This command completes asynchronously, since it depends on an externalevent, namely the acceptance of transmitted data by the attached serialdevice. An immediate command acceptance status is posted to the host CPU10 when the command is issued. The port number must be between zero andthe maximum number of ports supported by the host adapter 18'. If theport number is invalid or the port is cleared, an error code is returnedto the host. Otherwise an event pending completion status is returned:

    ______________________________________                                        R0             Immediate command status                                       ______________________________________                                    

When the output buffers are emptied, an interrupt is generated to thehost CPU 10 and a command completion status is passed to the host CPU10:

    ______________________________________                                        R0             command completion status                                      R1             port number                                                    R2, R3         host command reference                                         ______________________________________                                    

Wait For TTY Port Modem Status Change Command

This command notifies the host CPU 10 when the specified TTY portdetects a modem status change. The command has two parameters:

    ______________________________________                                        R0         Port number/Wait for TTY port status to                                       change                                                             R1, R2     Host command reference                                             ______________________________________                                    

This command completes asynchronously, since it depends on an externalevent, namely the acceptance of transmitted data by the attached serialdevice. An immediate command acceptance status is posted to the hostwhen the command is issued. The port number must be between zero and themaximum number of ports supported by the host adapter 18'. If the portnumber is invalid or the port is closed, an error code is returned tothe host. Otherwise an event pending completion status is returned:

    ______________________________________                                        R0             Immediate command status                                       ______________________________________                                    

When a modem status change is detected an interrupt is generated to thehost CPU 10. A command completion status is passed to the host:

    ______________________________________                                        R0             command completion status                                      R1             modem status                                                   R2, R3         host command reference                                         ______________________________________                                    

Cancel TTY Port Wait Command

This command cancels an asynchronous completion command, such as the"Wait for Output to Drain" command. The command has one parameter:

    ______________________________________                                        R0      Port Number/Cancel TTY port wait command                              ______________________________________                                    

The port number must be between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is closed, an error code is returned to the host CPU 10. Otherwisea successful completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Get TTY Port Status Command

This command returns the current status of the specified port. Thecommand has one parameter:

    ______________________________________                                        R0       Port number/Get TTY port status command                              ______________________________________                                    

The port number must be between zero and the maximum number of portssupported by the host adapter 18'. If the port number is invalid or theport is closed, an error code is returned to the host CPU 10. otherwisea successful completion status is returned:

    ______________________________________                                        R0             command completion status                                      R1             modem/line status                                              R2, R3         flow status                                                    ______________________________________                                    

DIRECT HIGH-SPEED CHANNEL COMMANDS

The following commands support use of a high-speed channel as a directlink between host adapters such as 18' in separate systems. There are nochangeable port characteristics associated with the high-speed channel.The link is a full-duplex 14-bit data pathway and provides throughput upto 300 kbytes per second.

Open Direct High-Speed Channel Command

This command changes the state of a specified channel from closed toopen. The command has two parameters:

    ______________________________________                                        R0         Channel number/Open channel command                                R1, R2     Host reference identifier                                          ______________________________________                                    

The channel number must be between zero and the maximum number of portssupported by the host adapter 18'. The maximum value is not necessarilyfixed, but may be obtained via the "Get Version Information" commanddescribed above. If the channel number is invalid or a port is alreadyopened to a data concentrator on the specified channel, an error code isreturned to the host. Otherwise, a successful completion status isreturned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

The host reference identifier is used in subsequent communications withthe host device driver. In operation, this is intended to be a pointerto a data structure maintained by the host CPU 10 which is related tothe specified channel. Its value and usage by the host CPU 10 aretransparent to the host adapter 18'.

Close Direct High-Speed Channel Command

This command changes the state of a specified channel from open toclose. The command has one parameter:

    ______________________________________                                        R0        Channel number/Close channel command                                ______________________________________                                    

The channel number must be between zero and the maximum number of portssupported by the host adapter 18'. The maximum value is not necessarilyfixed, but may be obtained via the "Get Version Information" commanddescribed above. If the channel number is invalid or a port is alreadyopened to the data concentrator 28 on the specified channel, an errorcode is returned to the host. Otherwise, a successful completion statusis returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

MISCELLANEOUS COMMANDS Shutdown All Ports and Channels Command

This command is equivalent to issuing close commands on all open TTYports and direct high-speed channels. The command has one parameter:

    ______________________________________                                        R0       Shutdown all ports and channels command                              ______________________________________                                    

Once all ports are closed, a completion status is returned:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

Execute Diagnostic Routine Command

This command provides an interface to the host adapter 18' firmwarediagnostic routines. Some of these diagnostic routines have side effectswhich limit their application once the host adapter 18' is initializedand in use by the driver. The command has at least two parameters:

    ______________________________________                                                   Diagnostic routine/execute diagnostic                              R0         routine command                                                    ______________________________________                                        0          ROM checksum test                                                  1          RAM read/write verify test                                         2          Timer interrupt test                                               3          Host interface mailbox test                                        4          Bus master block transfer test                                     5          High-speed channel internal                                                   loopback test                                                      6          High-speed channel external                                                   loopback test                                                      ______________________________________                                    

R1, R2, R3 Test parameters described below: The "RAM Read/Write VerityTest" the "Timer Interrupt Test" and the "Bus Master Block TransferTest" leave the host adapter 18' in a reset state. Any configurationinformation and buffered port data may be lost. The internal andexternal high-speed channel loopback tests leave the specified channelin a reset state. Any configuration and buffered port data may be lost.Additionally, the attached data concentrator 28 may be reset.

The command returns the result of the test routine:

    ______________________________________                                        R0           command completion status                                        R1, R2, R3   diagnostic routine completion status                             ______________________________________                                    

The "ROM Checksum Test" takes no additional parameters and returns thepass/fail status in R1:

    ______________________________________                                        R0            command completion status                                       R1            Pass/Fail                                                       ______________________________________                                         zero--pass                                                                    nonzero--fail                                                            

The "RAM Read/Write Verify Test" takes one additional parameter:

R1 Number of test repetitions

Since the entire contents of the RAM 42 are modified by this command thehost adapter 18' and firmware are left in a reset state on completion.Command and test completion status are returned as follows:

    ______________________________________                                        R0            command completion status                                       R1            Pass/Fail                                                       ______________________________________                                         zero--pass                                                                    nonzero--fail                                                            

The "Host Interface Mailbox Test" reads the test parameters from themailbox registers and uses them as described below to determine thevalues to be written back to the host CPU 10. Up to three parameters maybe given:

    ______________________________________                                               R0          Test value 1                                                      R1          Test value 2                                                      R2          Test value 3                                               ______________________________________                                    

The host adapter 18' determines which registers have been written by thehost CPU 10 and writes back the test value(s), with all bitscomplemented, to the same register(s). The test can be used to verifythe mailbox data register and the mailbox status register integrity. Ofcourse, at least the mailbox register R0 must be functioning in orderfor any command processing to take place. The host CPU 10 must verifyexpected register return values. Command and test completion status arereturned as follows:

    ______________________________________                                        R0            command completion status                                       R1            Test value 1 complemented                                       R2            Test value 2 complemented                                       R3            Test value 3 complemented                                       ______________________________________                                    

The "Bus Master Block Transfer Test" performs a simple data movementtest to verify correct operation of the bus master control logic 56. Thecommand takes two parameters:

    ______________________________________                                        R1, R2     Physical RAM address of host buffer                                R3         One to sixteen kbyte length of host buffer                         ______________________________________                                    

The host adapter 18' transfers the specified buffer from host RAM intothe RAM 42 and computes a 16-bit checksum. The contents of the bufferare complemented and the buffer is transferred back to the host RAM.Since the contents of the RAM 42 are modified by this command the hostadapter 18' is left in a reset state on completion. Command and testcompletion status are returned as follows:

    ______________________________________                                        R0            command completion status                                       R1            pattern checksum buffer                                         ______________________________________                                    

The "High-Speed Channel Internal Loopback Test" places the dataconcentrator 28 high-speed channel into software loopback mode. To enterand exit this mode, the data concentrator 28 must be reset, losingcurrent configuration and buffered device data. The command takes twoparameters:

    ______________________________________                                        R1             High-speed channel to test                                     R2             Number of test repetitions                                     ______________________________________                                    

Command and test completion status are returned as follows:

    ______________________________________                                        R0            command completion status                                       R1            number of loopback errors                                       ______________________________________                                    

The "High-Speed Channel External Loopback Test" places the hosthigh-speed channel into loopback mode. An external loopback connectormust be attached to the adapter channel under test before issuing thiscommand. The command takes two parameters:

    ______________________________________                                        R1             High-speed channel to test                                     R2             Number of test repetitions                                     ______________________________________                                    

If a loopback device is not attached, this test will fail and the dataconcentrator 28 and serial attached devices may receive unintended data.Command and test completion status are returned as follows:

    ______________________________________                                        R0            command completion status                                       R1            number of loopback errors                                       ______________________________________                                    

Peek Host Adapter RAM 42 Command

The Peek Host Adapter RAM (Debug/Patch) Command facilitates debuggingand patching of firmware in the host adapter 18'. This command allowsthe host device driver to examine arbitrary locations in the RAM 42address space. The command has two parameters:

    ______________________________________                                        R0           Peek host adapter RAM command                                    R1, R2       Physical RAM address                                             ______________________________________                                    

The work (16-bit) value at the specified location is returned to thehost CPU 10:

    ______________________________________                                        R0            command completion status                                       R1            data value                                                      ______________________________________                                    

Poke Host Adapter RAM 42 Command

The Poke Host Adapter RAM 42 Command also facilitates debugging andpatching of firmware in the host adapter 18' and allows the host devicedriver to write to arbitrary locations in the RAM 42 address space. Thecommand has three parameters:

    ______________________________________                                        R0          Poke host adapter RAM 42 command                                  R1, R2      Physical RAM 42 address                                           R3          data value                                                        ______________________________________                                    

The specified value is set immediately and a successful completionstatus is returned to the host CPU 10:

    ______________________________________                                        R0         command completion status                                          R1         previous contents of specified location                            ______________________________________                                    

This command is specified in such a way as to allow implementation ofsemaphores between the host adapter 18' and the host device driver. Thisfeature may be useful for debugging.

Load New Executable Image Command

This command allows for the loading of a new executable image into theRAM 42. Use of this command effectively stops operation of the currentcontrol program in the RAM 42. The new control program is downloaded,overwriting the previous program. The lower 1024-bytes of the RAM 42 arereserved for use during the download operation and may not beoverwritten by the download image.

The download image must begin with a special load control header, whichincludes such information as the destination address of the load image.The command has three parameters:

    ______________________________________                                        R0     Replace host adapter 18' control program command                       R1, R2 Physical address of load image in host RAM                             R3     Length of load image in bytes                                          ______________________________________                                    

The interrupt processing routine for the host adapter CPU 40 reads themailbox registers. On determination that the requested command is toload a new executable image, the host adapter CPU 40 aborts any busmaster transfer in progress and transfers the special load controlheader into a reserved location in the lower 1204-bytes of the RAM 42.The remainder of the load image is then transferred to the load addressspecified in the header. The interrupt vectors for the UART 46 isinitialized to point to "no-operation" (no-op) interrupt serviceroutines in the ROM 44. The interrupt service routine for the hostservice request interrupt is set up to point to a service routine in theROM 44. The segment registers and stack pointer are then set to thevalues specified in the control header. Prior to passing control to theentry point specified in the control header, a load command completionstatus is written to the mailbox register R0 and an interrupt is issuedto the host CPU 10 by writing to mail command register:

    ______________________________________                                        R0            command completion status                                       ______________________________________                                    

DEVICE I/O DATA TRANSFER PROTOCOL

Serial device input and output data is transferred between the host CPU10 and the host adapter 18' by the bus master control logic 56 asdirected by the host adapter CPU 40. The host adapter 18' requiresreserved, non-swapable data transfer buffers and control structures inthe address space of the host CPU 10. These data areas are provided andused cooperatively by the host device driver. If the host adapter 18' isa 16-bit ISA adapter, it can only drive the lower 24 address lines forbus master transfers when installed in an EISA system. Therefore, theshared data areas must reside within the lower 16-megabytes of host RAMaddress space.

The primary function of an intelligent serial controller is to reducethe number of interrupts to the host CPU 10 due to character input andoutput. The data transfer protocol described herein allows the hostadapter 18' to interrupt the host CPU 10 only as often as required toprovide acceptable user key input response. For higher-speed continuousdata stream connections the data transfer protocol results in receipt ortransmission of many characters per interrupt to the host CPU 10.

Another benefit of the data transfer protocol of the presently disclosedembodiment of the invention is to limit data copying while at the sametime isolating the host adapter 18' from knowledge of operating systeminternal data structures associated with serial devices. In the case ofUNIX device drivers, for example, the host adapter 18' may not directlymanipulate c-list data structures. For compatibility with multipleversions of the operating system and multiprocessor capabilities, suchcontrol is reserved for the device driver executed by the host CPU 10,and is in fact provided by function calls provided by the operatingsystem.

The data transfer protocol of the presently disclosed embodiment of theinvention is simple and efficient. The protocol uses four linear listdata structures as shall be hereinafter described. Indirect manipulationof complex data structures such as per device circular buffers in thehost address space might require many discrete transfer operations bythe bus master controller 56. The host device driver would still have tocopy the data into or out of the per device buffers from or to theoperating system internal data structures.

The locations, sizes, and other information regarding the shared datastructures is passed from the host device driver to the host adapter18'. The following data structure must be initialized by the driver andpassed to the host adapter 18' as a parameter to the "InitializeTransfer Data Structures" command:

    ______________________________________                                        D.sub.-- INTRPS                                                                             zero to twenty interrupts per second                            D.sub.-- IOSIZE                                                                             512 to 32768 bytes                                              D.sub.-- IOBUF                                                                              physical address                                                D.sub.-- CONTENTS                                                                           physical address                                                D.sub.-- RECVSPACE                                                                          physical address                                                D.sub.-- XMITSPACE                                                                          physical address                                                D.sub.-- OUTBAND                                                                            physical address                                                D.sub.-- PORTCOUNT                                                                          one to n ports                                                  ______________________________________                                    

The host device driver can use the first two parameters to control I/Olatency and memory usage. The number of interrupts per second (D₋₋INTRPS) determines how frequently the host adapter 18' will interruptthe host CPU 10 to transfer device I/O data. The I/O buffer size (D₁₃IOSIZE) determines how much data may be transferred for each interrupt.In general, the host adapter 18' should interrupt the host CPU 10 asinfrequently as keystroke response permits and with as much data aspossible. The numbers may be determined by experimentation, but aninterrupt frequency of ten times per second is probably sufficient toprovide good response. An I/O buffer size equal to the aggregatecombined device baud rate divided by the data character size and theinterrupt frequency should also be sufficient. For example, in oneembodiment of the present invention having an eight-port system with two38.4 kbaud devices, four 9600 baud devices and two 2400 baud devices,could use a 1200 byte I/O buffer at ten interrupts per second, asdetermined by the following computation:

    (2×38400)+(4×9600)+(2×2400)=120000 combined baud rate

    120000 bits per sec/10 bits per character=12000 characters per second

    12000 characters per second/10 interrupts per second=1200 characters per interrupt

The addresses of the data structures must be specified as physical RAMaddresses. Additionally, the host device driver must assure that thearea allocated to the data structures is locked-down, i.e. not swapable.The size of the I/O buffer is specified as described above. The size ofthe remaining data structures is determined by the number of ports whichthe host device driver supports on the host adapter 18'. This number maybe less than the maximum number of ports supported by the host adapter18'.

The host adaptor 18' may be adaptive in determining how often tointerrupt the host CPU 10. If more data is arriving during the specifiedinterrupt period than can be contained in the transfer buffer, then thehost adapter 18' may increase the frequency of interrupts up to amaximum of twenty per second. If the number of interrupts per second isspecified to be zero, the host adapter 18' has been configured tooperate without interrupts to the host CPU 10 and a polled mode ofoperation is selected. In this mode, the host CPU 10 has control of thefrequency of data transfers, as shall be hereinafter described.

Since both the host adapter 18' and the host CPU 10 may access andmodify the shared data structures, their use must be arbitrated. At anytime, the data structures are said to be "owned" either by the hostadapter 18' or the host CPU 10. The host CPU 10 owns the data structuressolely until it issues the command to initialize the data structures.Once this command has been executed, the model for sequencing access tothe data structures depends on whether interrupt driven or polled datatransfer operating mode has been selected.

For interrupt driven mode, the host adapter 18' may be thought to ownthe data structures, intermittently granting their control to the hostCPU 10 and expecting the host CPU 10 to yield them back in a timelyfashion. When the host adapter 18' has control of the data structures,the host CPU 10 may not modify their contents. Though the host CPU 10can read at any time the data structure contents without harm to thetransferred data, it may not interpret their contents to be coherentexcept when the host adapter 18' has granted control to the host CPU 10.

For polled mode, the host CPU 10 may be thought to own the datastructures, intermittently granting their control to the host adapter18' and expecting the host adapter 18' to yield them back in a timelyfashion. When the host CPU 10 has control of the data structures, thehost adapter 18' must not modify their contents. Though the host adapter18' can read at any time the data structure contents without harm to thetransferred data, it may not interpret their contents to be coherentexcept when the host CPU 10 has granted control to the host adapter 18'.

The data I/O buffer is the main structure for transferring data betweenthe host adapter 18' and the host CPU 10. It is organized merely as anarray of bytes of data to or from serial devices. Only one copy of theI/O buffer exists, that is the one in the address space of the host CPU10. Two copies of the other three data structures exist, one in the hostRAM and one in the RAM 42. The data I/O buffer is filled by the hostdevice driver with output data from the host computer to the serialdevices, and filled by the host adapter 18' for input data from theserial devices.

The data I/O buffer is an undistinguished array of bytes to or from anumber of devices. The contents list allows the host CPU 10 or the hostadapter 18' to determine for which device each byte of data in thebuffer is intended. The contents list is an array of device referencehandle and count entries, each entry having the following form:

    ______________________________________                                        CN.sub.-- TTYREF device reference handle                                      CN.sub.-- COUNT  byte count of device data                                    ______________________________________                                    

The end of the contents list is signified by a null value devicereference handle. The maximum length of the contents list is set by theport count parameter D₋₋ PORTCOUNT of the "Initialize Transfer DataStructure" command.

Two other data structures allow the host CPU 10 ultimately to controlinput flow and the host adapter 18' ultimately to control output flow.The host CPU 10 maintains a list of device reference handles andcounters which represent the available input buffer space in the hostfor each device. The beginning of this input buffer space list isindicated by the physical address sent by the host by the parameter D₋₋RECVSPACE. The host adapter 18' respects these limits and transfers nomore than the specified number of bytes for each device to the host CPU10 in the data I/O buffer. Similarly, the host adapter 18' maintains alist of device reference handles and counters which represent theavailable output buffer space in the host adapter 18' for each device.The beginning of this output buffer space list is indicated by thephysical address by the parameter D₋₋ XMITSPACE. The host CPU 10respects these limits and transfers no more than the specified number ofbytes for each device to the host adapter 18' in the data I/O buffer.

The final data structure contains per port out-of-band statusinformation such as break indications and gratuitous modem status changeindications. This data structure begins at the physical address D₋₋OUTBAND. Like the transfer buffer contents list, this data structure isan array of bytes that indicates which, if any, ports have anout-of-band status change or indication. Each entry is defined asfollows:

    ______________________________________                                        OB.sub.-- TTYREF TTY reference handle                                         OB.sub.-- TYPE   out-of-band status type                                      OB.sub.-- STATUS status value, as required                                    ______________________________________                                    

The end of this list is signified by a null value device referencehandle. The maximum length of this list is set by the port countparameter D₋₋ PORTCOUNT of the "Initialize Transfer Data Structure"command.

The following status types and status values are defined:

    ______________________________________                                        OBT.sub.-- BREAK                                                                             break character received                                                      (no associated status value)                                   OBT.sub.-- MODEM                                                                             modem status change                                                           (bit-mapped modem status value)                                OB.sub.-- STATUS                                                                             status value, as required                                      ______________________________________                                    

DESCRIPTION OF THE DATA CONCENTRATOR 28

The data concentrator 28 of FIG. 2 provides a facility forcommunications with up to eight asynchronous devices via the singlehigh-speed, full duplex data link 30. The communications facilitiesprovided by the data concentrator 28 include remote device I/O, flowcontrol, communications parameter configuration, and operational modeconfiguration. The maximum device rate for any individual device port inthe presently disclosed embodiment of the invention is 38.4 kbits persecond. The high-speed data link 30 between the data concentrator 28 andthe host adapter 18' in FIG. 2 operates at approximately 600 kbits persecond in each direction.

Turning now to FIG. 4, a block diagram showing the major functionalcomponents of one embodiment of the data concentrator 28 in accordancewith the present invention is provided. The data concentrator 28includes an RS-422 line driver 70 for interfacing with a correspondingone of the RS-422 line drivers 60 provided at the fanout device 64 ofthe host adapter 18'. The transmission of the data between the dataconcentrator 28 and the host adapter 18' is accomplished using theRS-422 differential current signalling conventions implemented by theline drivers 60 and 70. As a result, the data concentrator 28 may belocated as far as 500 feet from the host adapter 18'. The output sectionof the data concentrator 28 is provided by eight RS-232 line drivers,designated as 80 in FIG. 4, which are similar to the RS-232 line drivers58 in the host adapter 18'. The RS-232 line drivers 80 are connected toeight RJ-45 terminal connectors 82a-82h which are located on a fanoutdevice 64, wherein the RJ-45 terminal connectors 82a-82h are similar tothe RJ-45 jacks 66a-66h provided in the fanout device 64. RS-232compatible devices may thus be coupled either directly to the RJ-45jacks 66a-66h of the host adapter fanout device 64, or to the RJ-45modular jacks 82a-82h of the data concentrator 28.

In the presently disclosed embodiment, operation of the dataconcentrator 28 is controlled by a microcontroller 74 which may be acommercially available device, such as the HPC46004 high-performance20-MHz, 16-bit controller, manufactured by National Semiconductor. It iscontemplated that a faster (e.g. 30-MHz) microcontroller device may beemployed if higher performance is required. The microcontroller 74contains 512 bytes of internal RAM (not shown); in addition, the dataconcentrator 28 includes a 16k×16 EPROM 76 for storing start-uproutines, diagnostic routines, and control program code to be executedby the microcontroller 74. The data concentrator 28 further includes anoctal UART 78, which is preferably of the same type as the octal UART 46in the host adapter 18' and an HSRT 72 which is similar to and capableof communicating with any of the corresponding HSRT's 48-54 in the hostadapter 18'. Finally, the data concentrator 28 includes a memorycontroller and other support logic generally shown as support logic 84.

The memory map in the data concentrator 28 consists of the 512 byte RAMinternal to the microcontroller 74, the EPROM 76, and memory-mapped I/O.The physical addresses of all the data concentrator 28 hardware isdetermined by the implementation of conventional memory decoding logic(not shown). The physical addresses are referenced by the dataconcentrator 28 as public symbols that are defined to themicrocontroller 74 development system linker. The symbols and physicaladdresses for all memory-mapped I/O are defined in a single source codefile. The memory map is set forth in Table 12:

                  TABLE 12                                                        ______________________________________                                        DATA CONCENTRATOR 28 MEMORY MAP                                               Memory                                                                        Range     Type        Function                                                ______________________________________                                        FFFFh-8000h                                                                             Read only   ROM 76 firmware                                         7FFFh-7000h                                                                             Read/Write  Octal UART 78                                           6FFFh-6000h                                                                             Read only   Octal UART 78 interrupt                                 5000h     Read/Write  HSRT 72                                                 4000h     Read only   HSRT 72 status                                          3000h     Write only  Data terminal ready                                                           register                                                ______________________________________                                    

The first ninety-six words (192 bytes) of RAM in the microcontroller 74,referred to as "base" memory, are addressable in an efficient shortaddress form. Variables and data which must be frequently accessed bythe data concentrator 28 are allocated in this base page memory. Basepage addressing is an advantage only when using direct addressing modes.If base page memory is accessed indirectly, the access time andinstruction fetch time are identical to non-base-page accesses. The 512bytes of internal RAM in the microcontroller 74 are used for eightdevice output buffers, a host output buffer, a command response buffer,buffer counts and pointers, state tables, temporary variables, and astack. The data buffers in a ring buffer configuration with a headpointer, a tail pointer, and a count, which are all allocated in basepage RAM. Devices of various data communication rates may be attached tothe data concentrator 28. Buffering and flow control are provided in thedata concentrator 28 to reduce the host adapter 18' processingrequirements. Output buffers are defined as discrete entities to allowthe development system linker more flexibility in allocating the smallamount of system RAM. Thus some output buffers may exist in base pageRAM while others do not. Because device output buffers are alwaysaccessed indirectly, the RAM type chosen for the buffers does not affectthe execution efficiency of the data concentrator 28.

The HSRT's 48, 50, 52, and 54 in the host adapter 18' and the HSRT 72 inthe data concentrator 28, shown in FIG. 4, form the serialcommunications link between the host adapter 18' and the dataconcentrator 28. Each HSRT comprises a double buffered receiver sectionand a receiver buffer controller state machine, a double bufferedtransmitter section and a transmitter buffer controller state machine, areceiver state machine (FIG. 6), a transmitter state machine (FIG. 8)and a status register (not shown). The HSRT's 48, 50, 52, 54 and 72 aredesigned for a polled environment and have no interrupt capability.Referring to FIG. 2, recall that up to four separate data concentrators,each essentially identical to the data concentrator 28, can be connectedto the S0-S3 ports of the host adapter 18'. Therefore, each of theHSRT's 48, 50, 52 and 54 could be connected to a separate HSRT, similarto the HSRT 72, in a data concentrator similar to the data concentrator28. The HSRT located on a data concentrator is similar to the HSRT's 48,50, 52 and 54 on the host adapter 18', except that the HSRT's 48, 50, 52and 54 can initiate a reset sequence which commands the HSRT on the dataconcentrator 28 to enter a RESET state, whereas the HSRT 72 on the dataconcentrator 28 may not command the HSRT's 48, 50, 52 and 54 into aRESET state.

Since all HSRT's are similar, it would be instructive to describe atypical transmit and receive cycle between the HSRT 48 and the HSRT 72as indicative of all communication between the host adapter 18' and thedata concentrator 28. The hardware implementation of the RS-422 linedrivers 60 and 70 will be momentarily ignored such that there will onlybe four connection lines between the HSRT 48 and the HSRT 72: a receivedata line called RXDATA, a receive clock line called RXCLK, a transmitdata line called TXDATA, and a transmit clock called TXCLK. Note thatthe HSRT 48 signals RXDATA, RXCLK, TXDATA and TXCLK are connected to theHSRT 72 signals TXDATA, TXCLK, RXDATA and RXCLK, respectively, to form afull duplex communication link.

The HSRT 48 transmitter state machine indicates that data is ready to betransmitted by setting the signal TXDATA of the HSRT 48 low. The HSRTtransmitter state machine then polls its TXCLK signal to determine whenan acknowledge signal is sent. The HSRT 72 receiver state machine,meanwhile, polls its RXDATA signal (connected through the HSRT 48 TXDATAsignal) to see when it goes low, indicating that data is ready to besent. When the HSRT 72 receiver state machine (FIG. 6) detects itsRXDATA signal low and is ready to receive data, it outputs seventeensuccessive clock pulses on the signal RXCLK of the HSRT 72. The firstclock pulse is the acknowledge signal which signals the HSRT 48 tosupply valid data on the HSRT 48 TXDATA line. The remaining sixteenclock pulses from the HSRT 72 RXCLK signal to the HSRT 48 TXCLK signalcause valid data to be transferred from the HSRT 48 TXDATA signal to theHSRT 72 RXDATA line. This data is transmitted from a transmit shiftregister 352 (FIG. 9) in the HSRT 48 and is latched into a receivershift register 250 (FIG. 7) located in the HSRT 72. In a similar manner,the HSRT 72 transmitter state machine (FIG. 8) indicates that it isready to send data to the host adapter 18' by setting its TXDATA signallow. It then polls its TXCLK signal for an acknowledge signal. The HSRT48 receiver state machine polls its RXDATA signal until it goes lowindicating data is ready to be sent. The HSRT 48 receiver state machinethen transmits seventeen successive clock cycles on its RXCLK signal andreceives data on its RXDATA signal in the same manner as describedabove. Note that the receiving HSRT always provides the clock pulses tothe transmitting HSRT in order to clock in valid data. In this manner,the interface between the HSRT 72 and one of the HSRT's 48-54 providesfor an implicit level of flow control between the host adapter 18' andthe data concentrator 28.

The double buffered transmitter section (FIGS. 9 and 9A) on each HSRTconsists of a sixteen bit transmitter buffer 350, a sixteen bittransmitter shift register 352 and a transmitter buffer controller statemachine (FIG. 9A) to control the operation of the double buffer. Data iswritten to the transmitter buffer 350 by the local processor. When thetransmitter shift register 352 is empty, the transmitter buffer 350 ischecked to see if it has data. When the transmitter buffer 350 is fullof valid data, this data is latched into the transmitter shift register352 to be transmitted as described above. The transmitter buffer 350 isthen cleared to receive more data to be transmitted. The data in thetransmitter shift register 352 is first shifted left by two bits andappended with a parity bit and a stop bit. Odd parity is utilized andthe stop bit is always zero.

The double buffered receiver section (FIGS. 7 and 7A) consists of asixteen bit receiver shift register 250, a sixteen bit receiver buffer252 and a receiver buffer controller state machine (FIG. 7A) to controlthe operation of the double receiver buffer. The receiver buffer 252 ischecked for when it becomes empty. Data received by the receiver shiftregister 252 (from a remote HSRT) is shifted right two bits and thenlatched into the lower fourteen bits of the receiver buffer 252. If aparity error is detected, however, the new data is not latched into thereceiver buffer 252 and a PARITY ERROR FLAG is set. The PARITY ERRORFLAG is used as an indication of the integrity of the line between thehost adapter 18' and the data concentrator 28. Bit fifteen of thereceiver buffer 252 is referred to as the RXFULL flag. This flag is setwhenever new data is being latched into the receiver shift register 250and is cleared when data is read from this register. Bit fourteen of thereceiver buffer 252 is referred to as the TXFULL flag. The TXFULL flagis set whenever new data is being latched into the transmitter shiftregister 352 and is cleared when data is read from this register.

The HSRT status register is a three bit read-only register. Bit zero isthe RXFULL flag, bit one is the TXFULL flag and bit two is the PARITYERROR FLAG, and these are the same flags as described above.

The receiver and transmitter state machines will now be described alongwith the RESET state which commands the data concentrator 28 to enter areset sequence. An oscillator circuit (not shown) residing on the hostadapter 18' provides many of the clock signals used by the components onthe host adapter 18' such as the host adapter CPU 40, the octal UART 46and the ASIC. One clock signal, preferably operating at a frequency ofapproximately 9.83 MHz, is provided to the ASIC to clock the receiverand transmitter state machines and the receiver and transmitter buffercontroller state machines on the host adapter 18'. The clock signal ispreferably divided by ten within the ASIC to provide a 983 kHz statemachine clock. The 983 kHz state machine clock is preferably divided bytwo to provide the TXCLK signal for each of the HSRT's 48-54. The octalUART 78 residing on the data concentrator 28 provides a 9.83 MHz clock,which is preferably divided by ten to provide a 983 kHz clock which isused to clock the receiver and transmitter state machines as well as thereceiver and transmitter buffer controller state machines on the dataconcentrator 28. The 983 kHz clock on the data concentrator 28 isdivided by two to provide the TXCLK signal for the HSRT 72.

Referring now to FIG. 6, when the HSRT 72 is initially powered up,indicated by POWER ON RESET, the receiver state machine of the HSRT 72enters a state 200. In the state 200, 3 signals CNTCLR*, AHCLR*, andPARRST* are asserted active low. The CNTCLR, signal resets a bitcounter, the bit counter being used to count the number of bitsreceived. The AHCLR* signal is a safety mechanism to prevent a signalAHRST (described below) from being asserted accidentally or from noise,and the PARRST, signal resets a parity checking circuit. The receiverstate machine enters a state 202 where it polls the RXDATA signalwaiting for it to go low. In the state 202, the AHCLR* and CNTCLR*signals remain asserted low. If the RXDATA signal goes low, this signalsthe receiver state machine that the HSRT 48 is ready to transmit data tothe HSRT 72. When the RXDATA line does go low, the receiver statemachine enters a state 204 where the CNTCLR* and PARRST* signals areasserted low. The receiver state machine steps through several delaystates 206, 208, 210, 212 and 214, monitoring to see if the HSRT 48 issending a reset sequence. The reset sequence occurs during this time ifthe HSRT 48 transmits two rising edges on its TXDATA line before theacknowledge signal is transmitted by the HSRT 72 to the HSRT 48. In thestate 206, the PARRST* signal is asserted, and in the states 206-214,the CNTCLR* signal is continually asserted. If the HSRT 72 detects thetwo rising edges on its RXDATA line, then the signal AHRST is assertedhigh and the receiver state machine enters a state 216 and the dataconcentrator 28 enters a sixteen microsecond RESET state.

The AHCLR* signal is not asserted in the states 204-214 to allow forAHRST signal to be asserted. If the AHRST signal is asserted, the statemachine enters the state 216 and a signal BHRST* is asserted low toinitiate a timer (not shown) to begin the sixteen microsecond reset. Thesignals AHCLR* and CNTCLR* are also asserted in the state 216. The statemachine then re-enters the state 200. The receiver state machines forthe HSRT's 48-54 on the host adapter 18' are similar to this statemachine, except that the receiver state machine for the HSRT's 48-54 donot enter the state 216 since the data concentrator 28 can not send areset sequence to the host adapter 18'. This is the primary differencebetween the receiver state machine for the HSRT 72 on the dataconcentrator 28 and the receiver state machines for the HSRT's 48-54 onthe host adapter 18'.

In the state 214, if a reset sequence was not sent such that the AHRSTsignal remains low, the receiver state machine polls the RXFULL flag todetermine when the receiver shift register 252 is ready to accept data.When the RXFULL flag is cleared indicating that the receiver shiftregister 252 is empty, and the AHRST signal is low, the receiver statemachine enters a state 218. In the state 218, the CNTCLR* signal remainsasserted, and the AHCLR, signal is once again asserted to prevent afalse reset state. An RXENABLE* signal is asserted low in the state 218which enables the receiver shift register 252 to receive data. Thereceiver state machine then steps through a state 220 and into a state222. The AHCLR*, CNTCLR* and RXENABLE* signals remain asserted in thestate 220, which is essentially a timing delay state.

In the state 222, the HSRT 72 asserts the clock signals on the RXCLKline to latch sixteen bits of data into the receiver shift register 252.A signal RXCOMPLETE remains low until all the data is received, which isindicated by the bit counter. The RXCOMPLETE signal is polled in thestate 222, and the receiver state machine enters a next state 224 whenthe RXCOMPLETE signal goes high. The AHCLR* and RXENABLE* signals remainasserted in the state 222, but note that the CNTCLR* signal is notasserted since the bit counter counts the number of bits received whilein the state 222 to determine when all the data is received. When thebit counter reaches a certain pre-defined number to indicate the numberof bits to be received, the RXCOMPLETE signal is asserted high and thereceiver state machine enters the state 224. The CNTCLR* signal isre-asserted in the state 224 to clear the bit counter circuit, and theAHCLR* signal remains asserted low.

The receiver state machine then enters a state 226 in which a signalRXLATCH is asserted high. The RXLATCH signal clocks a parity logginglatch (not shown) which indicates whether a parity error has occurred.If a parity error has occurred, a signal RXFULL will not be assertedhigh (described below), such that the data in the receiver shiftregister 250 is considered invalid and will not be latched into thereceiver buffer 252. The data in the receiver shift register 250 iseventually discarded as new data is shifted into the receiver shiftregister 250 during the next cycle if a parity error has occurred. If noparity error is detected, then the signal RXFULL will be asserted andthe data is eventually latched into the receiver buffer 252.

The AHCLR* and CNTCLR* signals are also asserted in the state 226. Thestate machine then enters a state 228 wherein the RXDATA signal ispolled until it goes high, to prevent a bad data transfer from takingplace if the RXDATA signal has not returned to a high state by the timethe RXCOMPLETE signal has been generated. The AHCLR* and CNTCLR* signalsare asserted in the state 228. When the RXDATA signal goes high, thereceive cycle is complete, and the receive state machine re-enters the202 state to monitor when the RXDATA signal goes low again for a newreceive cycle.

Referring now to FIGS. 7 and 7A, the double buffered receiver sectionand buffer controller state machine within each of the HSRT's 48-54 and72 will be described. In FIG. 7, the receiver shift register 250receives data from the RXDATA signal input. The data is clocked in, onebit at a time, with the RXCLK input. When the receiver shift register250 is full of valid data, the signal RXFULL is asserted high. If aparity error has occurred, the RXFULL signal will not be asserted high.If the data is valid, the data is transferred to the receive buffer 252through a data path 254 when a signal LOADY* is asserted low. A signalYFULL goes high when the receiver buffer 252 is full of data. Thereceiver shift register 250 can be cleared to receive new data byasserting a CLRX* SIGNAL.

The receiver buffer controller state machine that controls the operationof the receiver shift register 250 and receiver buffer 252 is shown inFIG. 7A. Recall that the 983 kHz clock provides the clock which clocksthe receiver buffer controller state machine. Upon assertion of theRESET* signal low, the state machine enters a state 260 wherein theCLRX* signal is asserted to clear the receiver shift register 250. Thestate machine then enters a state 262 wherein the YFULL signal is polledwhile the receiver buffer 252 is still full of valid data. When thelocal CPU reads from the receiver buffer 252, the YFULL signal goes lowand the state machine enters the state 264. Meanwhile, after thereceiver shift register 250 was cleared, it could receive new data. Thereceiver buffer controller state machine polls the signal RXFULL untilit goes high indicating that the receiver shift register 250 is full ofvalid data. If valid data is received, the state machine enters a state266 wherein the signal LOADY, is asserted to transfer data from thereceiver shift register 250 through the data path 254 into the receiverbuffer 252. The state machine then re-enters the state 260 wherein thesignal CLRX* is asserted once again to clear the receiver shift register250 to receive new data. In the state 264, if a parity error isreceived, the RXFULL signal will not go high and the receiver buffercontroller state machine remains in the state 264. The receiver statemachine meanwhile enters another receive cycle to attempt to latch invalid data. When valid data is eventually received, the RXFULL signal isasserted and the receiver buffer controller state machine enters thestate 266. The double buffered receiver section and state machine thuscontinually moves valid data away from the receive shift register 250into the receive buffer 252 so that new data can be received by thereceiver section of the local HSRT which helps keep the receiverefficiency high.

Referring now to FIG. 8, the transmitter state machine of the HSRT 48will now be described. Recall that the 984 kHz clock within the ASICclocks the transmitter state machine of the HSRT 48. In a first state300, the transmitter state machine polls the signal TXFULL to determinewhen the transmitter shift register 352 is full of valid data to besent. When the transmitter shift register 352 is full of valid data,TXFULL goes high and the transmitter state machine enters a state 302,where it asserts a signal TXENABLE, to enable the transmitter shiftregister 352 to serially shift data out to the HSRT 72. While in thestate 302, the transmitter state machine waits for a rising edge on itsTXCLK signal, which is indicated by a signal TXEDGE and is sent by theremote HSRT 72 to indicate that the HSRT 72 is ready to receive data.The signal TXEDGE is asserted high every time a rising edge is receivedon the TXCLK signal.

When the TXEDGE signal goes high, the transmitter state machine enters astate 304 and the transmitter shift register 352 begins to seriallyshift data out to the remote HSRT 72 with each rising edge of the TXCLKsignal. A signal EDGERESET* is asserted low which resets the TXEDGEsignal low. The signal TXENABLE* remains asserted low in the state 304.The state 304 and the next three states 306, 308, and 310 formessentially a delay loop which monitors the TXCLK and TXEDGE signals todetermine when the remote HSRT has completed clocking in the transmitteddata. Recall that the TXCLK clock frequency is preferably half thefrequency of the transmitter state machine clock. The state machinesteps through the states 304-310 with each pulse of the clock, assertingthe signal TXENABLE* while data is being transmitted. In the state 310,the signals TXCLK and TXEDGE are monitored to determine whether thetransmission is complete. In the state 310, if the TXCLK signal is lowand TXEDGE is low, or if a rising edge has recently been received suchthat the TXEDGE signal is high, then data transmission is not completeand the transmitter state machine will loop back into the state 304.Again, in the state 304, the signal EDGERESET* is asserted to clear theTXEDGE signal so that the next rising edge, if any, on the TXCLK signalcan be detected.

Once the data transfer is complete, the receiving HSRT 72 holds theTXCLK signal high for a predetermined period of time to indicate thecompletion of transmitted data. The complete condition will be detectedby the state machine in the state 310 by the TXCLK signal being high(logic 1) and the TXEDGE signal being low (rising edge not detected) andthe transmitter state machine will enter a state 312. In the state 312,the EDGERSET* signal is asserted to clear TXEDGE, and a signal TXCLEAR*is asserted low to clear the transmitter shift register 352 (FIG. 9) toprepare it to receive more data for transmission. Also, if the RESET,signal is asserted low when the data transfer is complete, thetransmitter state machine will remain in the state 312 until the RESET,signal is de-asserted high. If the RESET, signal is not asserted, orwhen it is finally de-asserted high, the transmitter state machinere-enters the state 300 and the process repeats.

Referring now to FIGS. 9 and 9A, the double buffered transmitter sectionwill be described. In FIG. 9, the transmitter buffer 350 receives datafrom the local CPU to be transmitted to the remote HSRT. The signalAFULL is asserted high when the transmitter buffer 350 is full of validdata. The data is transferred to the transmitter shift register 352through a data path 354 when a signal LOADTX* is asserted low. Thetransmitter buffer 350 can then be cleared by asserting a signal CLRA*low so that it can be loaded with new data to be transmitted. The signalTXFULL is asserted high when the transmitter shift register 352 is fullof data to be transmitted. Data is shifted out on the TXDATA signal, onebit at a time, upon successive clock pulses asserted on the TXCLKsignal. When all the data is sent, the TXFULL signal goes low indicatingthat the transmitter shift register 352 is ready to receive more datafrom the transmitter buffer 350.

The transmitter buffer controller state machine that controls theoperation of the double buffer transmitter section is shown in FIG. 9A.Again, the 983 kHz clock clocks the transmitter buffer controller statemachine. When the RESET* signal asserted low, the state machine enters astate 360 wherein the CLRA* signal is asserted low to clear the innertransmitter buffer 350. The state machine then enters a state 362wherein the TXFULL signal is polled until it goes low indicating thatthe transmitter shift register 352 is empty and ready to receive moredata. When the TXFULL signal goes low, the state machine enters a state364 wherein the AFULL signal is polled to determine when it goes high.When the AFULL signal is asserted high, the transmitter buffer 350 isfull of data received from the local CPU to be transmitted to the remoteHSRT. The state machine then enters a state 366 wherein the signalLOADTX* is asserted to transfer data from the transmitter buffer 350 tothe transmitter shift register 352 through the data path 354. Thetransmitter state machine, described previously, controls thetransmission of data out of the transmitter shift register 352. Thetransmitter buffer controller state machine then re-enters the state 360wherein the CLRA* signal is asserted once again to clear the transmitterbuffer 350 to receive more data from the local CPU. Therefore, thedouble buffered transmitter section continually moves data away from thetransmitter buffer 350 into the transmitter shift register 352 so thatnew data can be transmitted by the local HSRT which helps keep thetransmitter efficiency high.

ADDRESSED PACKET PROTOCOL

All communication between the host adapter 18' and the data concentrator28 is through the HSRT 72 using a fourteen-bit addressed packetprotocol. The fourteen-bit packet, or frame, provides for multiplexingand demultiplexing information, flow control information, diagnostic andstatus massaging, device data, and configuration information. Ingeneral, the frame consists of a three bit frame-type field, a three bitaddress field, and an eight bit data field. The frame-type fieldindicates what type of information is contained in the frame. In mostcases, the address field indicates the logical port number (i.e. whichof the RJ-45 connectors 82a-82h that the device is connected to) forwhich the data applies. The exception to this rule are frames which haveglobal meaning to the host adapter 18' or the data concentrator 28. Thedata field is context dependent, according to the frame-type. Detailsare described below.

The UART 78 supports eight full-duplex asynchronous channels whichprovides the communication link between the data concentrator 28 and theasynchronous devices. Each of the eight channels of the UART 78comprises a transmitter and a receiver that can be programmed for any ofthe standard baud rates between 110 and 38.4K. Each channel contains 42bytes of First-In-First-Out (FIFO) buffer space divided into threefourteen-bit buffers using the fourteen-bit addressed packet protocolincluding an eight-byte transmit FIFO, an eight-byte receive FIFO and aneight-byte status FIFO. Each channel also supports the following: Userprogrammable and automatic flow control modes; four modem controlsignals--CD, DSR, RTS and CTS; odd, even, no parity or forced parity; 1,1.5 and 2 stop bits; and five to eight character bits plus optionalparity.

As shown in Table 12 above, the UART 78 functions on the microcontroller74 system bus as a memory mapped device. The UART 78 uses the systemsread, write, address, data and interrupt lines. The support logic 84supplies the chip select and interrupt acknowledge signals for theUART78. The microcontroller 74 supports four system level interrupts sothat the UART 78 can interrupt the microcontroller 74 when necessary.The first level interrupt is nonmaskable and is asserted when aninterrupt service to the UART 78 has been unsuccessful. The second levelinterrupt is asserted by the UART 78 when any of the UART 78's receivedata FIFO buffers reaches its preprogrammed threshold. The third levelinterrupt is asserted by the UART 78 when any of the UART 78's transmitdata FIFOs are emptied or the transmitter holding and shift registersare emptied. The fourth level interrupt is asserted by the UART 78 whenany of the UART 78's modem control signals change state.

The data concentrator 28 generates output delays on a per channel basis.Output delays are timed by the UART 78. The UART 78 is programmed for amode of operation which allows for embedded transfer control viasequences in the data stream. When the data concentrator 28 is sent anoutput delay frame, the microcontroller 74 builds an escape sequence inthe appropriate device output buffer. Generation of break signals isaccomplished in a similar manner.

The data concentrator 28 is responsible for executing, and in mostcases, responding to commands as defined in the addressed packetprotocol. In general, the data concentrator 28 will respond to allcommands except those which affect the immediate flow state of a devicechannel. The data concentrator 28 imposes a limit on the frequency ofcommands that the host adapter 18' and also imposes a limit of oneactive command per channel, except for a few special cases, such as flowcontrol commands and line status requests. Commands which requireacknowledgement are acknowledged via the addressed packet protocol. Ifthe host adapter 18' violates the one active command in progressrestriction, the data concentrator 28 issues a command-in-progress framevia the addressed packet protocol. Command responses are placed in thecommand response buffer when the command is processed. Command responsesand other information sharing the command response buffer are sent tohost adapter 18' by the host transmit prioritization of the polling loopwhich will be described below.

The data concentrator 28 may transmit status information to the hostadapter 18' via the addressed packet protocol. The status informationmay be sent either at the request of the host adapter 18' or as a resultof a monitored status change. Status change information will be sent tothe host adapter 18' when a monitored state changes or a monitored eventoccurs. The host adapter 18' may select the monitored conditions via theaddressed packet protocol.

When the host adapter 18' transmits data intended for an asynchronousdevice attached to the data concentrator 28, the data concentrator 28receives the data from the HSRT 72 during its polling routine. Thedevice for which the data is intended is determined at that time, andthe data is placed in the appropriate device output buffer. It is theresponsibility of the device output flow control mechanism to ensurethat the host adapter 18' does not transmit data intended for a devicewhich has no remaining buffer room. Device data is taken from the deviceoutput buffers and passed to the device during the transmit interruptservice routine which will be described below.

Data is received from devices during the receive interrupt serviceroutine. When the data is received from the device, a data frame isassembled according to the addressed packet protocol and immediatelyplaced in the host adapter output buffer. The device input flow controlmechanism is responsible for preventing the host adapter output bufferfrom becoming full. The data is passed on to the host adapter 18' by thedata concentrator 28 polling routine.

The primary function of the data concentrator 28 main polling loop is toroute data between the HSRT 72 and the data buffers, and to initiateexecution of command processing routines. The polling routineperiodically checks the HSRT 72 data and status registers for validincoming data. When a data frame is received, the polling routinedetermines the frame type from the frame-type field in the packet andtakes appropriate action. If the data received is intended for a device,it is buffered immediately. All other frame types are processed by thecommand processing routines. Device data, command responses and flowcontrol information share the HSRT 72. Prioritization of the above datatypes is managed by the host transmit prioritization mechanism of thepolling routine. The transmit priority is fixed in descending order offlow halting, command responses, and device data.

As described previously, to facilitate automatic detection of the dataconcentrator 28 by the host adapter 18', the data concentrator 28guarantees the transmission of an ID response packet, consistent withthe addressed packet protocol, on a periodic basis. The dataconcentrator 28 accomplishes this by transmitting a unique frame whenthe transmitter has been idle for a significant period of time.

Four interrupt types are serviced in the data concentrator 28 firmwarein the following fixed priority: Receive data (highest priority),transmit data, modem control signal change, and timer (lowest priority).The UART 78 is programmed to interrupt the microcontroller 74 whenvarious conditions occur. The specific interrupt conditions are dictatedby configuration packet protocol frames from the host adapter 18'. Forexample, if the host adapter 18' configures the data concentrator 28 toreport modem control signal changes, the UART's 78 modem changeinterrupt is enabled.

The receive data interrupt routine is divided into good data andexception data routines. The UART 78 indicates which type of receiveddata interrupt has occurred. In the case of good data, the UART 78supplies the channel number and a count of the characters in the FIFO ofthe UART 78. The good data receive service routine reads the charactersfrom the data register of the UART 78, assembles data frames accordingto the addressed packet protocol specification, and places the data inthe host adapter 18' output buffer. The service routine then checks theremaining room in the buffer, and if the buffer is sufficiently full,flow control for all devices is initiated as described below.

In the case of exception data, the type of exception is compared tointernal state tables to determine if the host adapter 18' is concernedwith this type of exception for the channel which caused the interrupt,or if the data concentrator 28 is to perform some other action based onthe exception condition, such as flushing input buffers on break detect.If the state information indicates that the condition should be reportedto the host adapter 18', such as modem control signal changes, a frameis constructed according to the addressed packet protocol and placed inthe host adapter output buffer. If the exception condition is not to bereported, the exception condition is ignored.

In most operating modes of the data concentrator 28, the UART 78 isprogrammed to interrupt when the transmitter FIFO becomes empty. TheUART 78 will then generate interrupt requests as soon as the lastcharacter in the transmit FIFO is written to the transmit shiftregister. When the interrupt is serviced, the UART 78 supplies theinterrupting channel number, and implicitly indicates that it is readyto accept up to eight characters for that channel. The device outputbuffer for the interrupting channel is checked for data to transmit. Ifdata is available, characters are written to the transmit register untileight characters have been written, or until the device output bufferbecomes empty.

If the output buffer is empty at the time the interrupt occurs, internalstate tables are examined to see if the host adapter 18' had requested asignal when the output buffer became empty. If a wait for output bufferempty request had been made for the interrupting channel, the commandprocessing routine would have set the interrupt conditions in the UART78 to request an interrupt on transmitter empty. If the wait for outputbuffer empty state is active, the occurrence of this interrupt indicatesthat the last character written to the data register has been serializedand shifted out to the device. When this occurs, a frame is placed inthe host adapter output buffer indicating that the output buffer hasemptied. The data concentrator 28 then resets the transmit interruptconditions in the UART 78 to interrupt on transmit FIFO empty, andclears the wait for output buffer empty state.

For modem control signal change interrupts, the UART 78 is set tointerrupt on the conditions in which the host adapter 18' is interested,or for which the host adapter 18' has requested flow control. When theinterrupt occurs, the cause of the interrupt is compared to statusinformation to determine if flow control action is required, and ifnecessary, the transmitter is enabled or disabled according to the typeof signal change that occurred. If the change is one that the host hasexpressed interest in, a frame is constructed according to the addressedpacket protocol, and placed in the device input/host output buffer.

A timer interrupt has two potential timer sources which may interruptthe microcontroller 74. One of the timer interrupts is enabledimmediately after the memory tests, and remains enabled thereafter. Thefunction of this timer is to generate a time base for servicing themicrocontrollers 74 watchdog logic. This watchdog timer is merely afail-safe mechanism in case the microcontroller 74 program crashes.Normally, the watchdog timer will be serviced if the microcontroller 74is executing properly. Otherwise, if the watchdog timer is not serviced,it triggers a hardware reset of the entire data concentrator 28. Thesecond timer interrupt serves as a time base for a signaling timeoutmechanism, and is enabled by a call to the settimer() function in thedata concentrator 28 firmware. The settimer() function accepts a pointerto a semaphore variable and a time interval as arguments. The settimer()function sets the second interrupting timer to the specified intervaland enables the second timer interrupt. When a timer interrupt occurs,the timer interrupt service routine polls the timer registers todetermine the source of the interrupt. If the watchdog service timerinterrupt is pending, the watchdog timer is serviced before checking thestate of the second timer. If the second timer interrupt is pending, thesemaphore is updated, and interrupts for the signal timer are disabled.

FLOW CONTROL

Device input flow control, that is, information from the device flowingto the host, is under the control of the host adapter 18' during normaloperation of the data concentrator 28. Note also, however, that the hostalso exercises input flow control by defining the maximum number ofbytes to be received from the host adapter 18' from each device in anygiven transfer of data to the data structure beginning at the addressidentified by D₋₋ RECVSPACE. When the host adapter 18' can not readilyaccept data from a device, it must initiate flow control via theaddressed packet protocol. This flow control may be in-band,out-of-band, or both. Flow control information is then immediatelypassed on to the device by the data concentrator 28. The dataconcentrator 28 executes flow control commands from the host adapter 18'but does not acknowledge them as it does other types of commands. Flowcontrol commands from the host adapter 18' therefore, are executedwithout any acknowledgement whatsoever.

The host adapter 18' has the responsibility of preventing overflow ofthe host output buffer in the data concentrator 28. To prevent overflowof the buffer, the host adapter 18' must coordinate input pollingfrequency with input flow control. If the host adapter 18' is not ableto poll the receive buffers of the HSRT's 48, 50, 52 or 54 at a rategreater than the aggregate character input rate of all eight devices, itmust initiate flow control on a per-channel basis to reduce the inputrate to a manageable level. If this requirement is not met, the dataconcentrator 28 will enter a global flow control state, to be describedbelow, in which it attempts to halt all input flow.

As described previously, 36K bytes of the RAM 42 are reserved for deviceinput and output buffers. Each input buffer and output buffer isimplemented in first-in-first-out (FIFO) form for each of the devices12-16 in the system. The host adapter CPU 40 takes data from the devices12, 14, and 16, either received from the HSRT's 48-54 or from the octalUART 46, and places the data into a device specific input buffer to belater transferred to the data I/O buffer located on the host. Similarly,the host adapter CPU 40 takes data from the device specific outputbuffers, determines where in the system the device is located, and sendsthe data to the device. For example, if the data is intended to go tothe device 12, the host adapter CPU 40 assembles an addressed packet andtransfers the data to the HSRT 48 to eventually be transferred to thedata concentrator 28, and ultimately to the device 12. If the data isfor the device 16, the data is transferred to the appropriate UARTlocated in the octal UART 46 to be transferred across the serial link 26to the device 16.

A standard FIFO implementation, as known to those skilled in the art,includes a count identifying the number of bytes contained in the FIFObuffer. Each time the host adapter CPU 40 inputs data into an inputbuffer, it updates the count number and compares the number with apredetermined high level number indicating FIFO overflow. If the amountof data in the FIFO is above the predetermined high level, the hostadapter CPU 40 sends a flow command to the corresponding deviceassociated with the input FIFO buffer, instructing the device to stopsending data. Eventually, the host adapter CPU 40 and the bus master 56will take data out of the input FIFO buffer and bus master the data tothe host computer data I/O buffer. The host adapter CPU 40 decrementsthe count of data in the input FIFO buffer and compares the new countwith a predetermined low level. If the actual count is at or below thepredetermined low count level, the host adapter CPU 40 re-initiates flowfrom the device by sending the device another flow command, instructingthe device to begin sending data again.

As described above, the host adapter 18' initiates flow control on aper-channel basis to keep data flow at a manageable level. If, however,the host adapter 18' is unable to clear the host output buffer in thedata concentrator 28, the data concentrator 28 initiates a fail-safemechanism called global flow control. If the data concentrator 28 entersthe global flow control state, it initiates flow control on all channelsaccording to the current flow state of each channel. The dataconcentrator 28 maintains the current input flow state of each device asit processes flow control commands from the host adapter 18'. Inbandflow control for a channel is initiated by the data concentrator 28 ifthe host adapter 18' has enabled in-band flow control for that channeland that channel is not being controlled by the host adapter 18'. If thehost adapter 18' has enabled out-of-band global flow control for achannel, the modem control signal lines for that channel are negated.

When the main polling loop of the data concentrator 28 firmwaretransmits device data to the host adapter 18', it checks the host outputbuffer capacity. If global flow control is active and the buffer hasemptied sufficiently, the global flow control state is cleared, and theflow state of all devices is restored according to the current flowstate of each channel. Since the host adapter 18' may still issue flowcontrol commands to the data concentrator 28 while global input flow isactive, the data concentrator 28 maintains the state of flow control"ownership" for devices. If a device is not in the flow-active statewhen global flow control occurs, the flow control is owned by the globalflow mechanism. If the host adapter 18' sends a flow-off frame to adevice under global flow control, the host adapter 18' gains ownershipof flow control for that device. If the host adapter 18' sends a flow-onframe for a device when global flow control is active, the global flowmechanism regains ownership of flow control.

Device output flow control, that is, data flowing from the host to allof the devices, occurs on a perchannel basis and is managed at twopoints of data flow. The first is data flowing from the host adapter 18'to the data concentrator 28. This flow control prevents the deviceoutput buffers in the data concentrator 28 from overflowing. The secondpoint of data flow which is controlled is data flowing from the dataconcentrator 28 to the device. This type of flow control prevents thedata buffers in the device from overflowing.

Flow between the host adapter 18' and the data concentrator 28 isinitiated by the data concentrator 28 when a device output buffer beginsto fill. The main polling routine examines the buffer capacity at thepoint when device data is being put in a device output buffer. If thebuffer is full beyond a defined point, a flow control pending state isset for that channel. This flow-pending state is later examined by thehost adapter 18' transmit prioritization mechanism of the polling loop,and if the channel is in the flow-pending state, a flow-off controlframe is sent to the host adapter 18' and the flow-pending state ischanged to flow-active state. But before the flow-active state isentered, the flow-pending state can be cleared by the transmit datainterrupt service routine. If the buffer is sufficiently emptied toanother predefined point by the transmit service routine during theflow-pending state, the state will be cleared. Otherwise, theflow-active state will be entered.

The flow-active state for a channel is also cleared by the transmitinterrupt service routine. When the device output buffer is sufficientlyemptied by the transmit service routine, the flow-active state iscleared, and a flow-on frame is put in the command response buffer. Thecontents of the command response buffer are sent to the host adapter 18'by the host transmit prioritization mechanism of the polling loop.

Device output flow between the data concentrator 28 and the device isregulated by the device. The data concentrator 28 may be configured on aper-channel basis by the host adapter 18' to recognize various forms offlow control. If a channel is configured for in-band flow control, theflow control characters must also be configured. Once configured,in-band flow control is processed by the UART 78, and no processing bythe microcontroller 74 is required. A channel may also be configured forout-of-band flow control. The input signals CTS and DSR may be used toenable or disable the transmitter for a device. If both CTS and DSR areconfigured in this auto-transmitter enable mode, a state change oneither modem control signal will result in the state of the transmitterbeing set according to the most recent signal change.

THE HOST ADAPTER 18' AND THE DATA CONCENTRATOR 28 ADDRESSED PACKETPROTOCOL

As described above, the packet, or frame, may comprise a frame-typefield, an address field if appropriate, or a data field if it is a datapacket. The packet may also comprise a frame-type field and a commandwherein the command instructs the data concentrator 28 to perform someoperation or function, if the frame is sent by the host adapter 18' tothe data concentrator 28. The data concentrator 28 also sends commandresponses back to the host adapter 18' through the addressed packetprotocol. The complete specification for the addressed packet protocolexists as a C language header file to be used by the host adapter 18'and the data concentrator 28 source modules. The frame-types and adescription of each is listed below.

The following frame-types are sent by the host adapter 18' to the dataconcentrator 28:

    ______________________________________                                        LHCT.sub.-- DEVDATA                                                                        A device data frame. The data is                                              intended for a device.                                           LHCT.sub.-- RCOND                                                                          Selects the conditions for which                                              the data concentrator 28 is to                                                transmit state change information.                               LHCT.sub.-- SBR                                                                            Sets baud rate of a device port and                                           enables or disables in-band flow                                              control.                                                         LHCT.sub.-- LPARM                                                                          Sets the operating parameters (Data                                           bits, parity, and stop bits) for a                                            device port.                                                     LHCT.sub.-- SETXON                                                                         Sets the XON flow control character                              LHCT.sub.-- SETXOFF                                                                        Sets the XOFF flow control                                                    character                                                        LHCT.sub.-- CMD                                                                            Selects diagnostic and other                                                  operating states. The data field                                              selects the specific state.                                      LHCT.sub.-- FLS                                                                            Performs flow state setting                                                   functions, selectable by a subset                                             of the data field.                                               ______________________________________                                    

The flow state setting function types are:

    ______________________________________                                        LHC.sub.-- FLOWX                                                                           Sets the device output flow state.                                            (sends XON or XOFF to a device).                                 LHC.sub.-- SETLINE                                                                         Sets modem signal line output                                                 states.                                                          LHC.sub.-- TXDELAY                                                                         Inserts an output delay in the                                                device output data stream.                                       LHC.sub.-- TXBREAK                                                                         Sends a break signal to a device.                                LHC.sub.-- SETAF                                                                           Sets auto hardware flow states in                                             the data concentrator 28.                                        ______________________________________                                    

The following frames are sent by the data concentrator 28 to the hostadapter 18':

    ______________________________________                                        LCHT.sub.-- DEVDATA                                                                        A device data frame.                                             LCHT.sub.-- STCHG                                                                          Reports status change information                                             for a device port.                                               LCHT.sub.-- PE                                                                             Reports a parity error for a device                                           port.                                                            LCHT.sub.-- LS                                                                             Reports modem control signal line                                             change.                                                          LCHT.sub.-- DRESP                                                                          Response to a mode setting command.                              LCHT.sub.-- FC                                                                             Flow control frame. Sets state of                                             host adapter 18' to data                                                      concentrator 28 flow.                                            LCHT.sub.-- RES                                                                            Response to reset (self test                                                  diagnostic result), or firmware                                               version request.                                                 ______________________________________                                    

When a valid frame is received by the data concentrator 28, an index iscreated from a combination of the frame-type and the address fields ofthe addressed packet protocol. This index accesses an entry in a receivehost data jump table of the firmware. The firmware routines accessed viathe jump table perform the following actions according to frame-type:

    ______________________________________                                        LHCT.sub.-- DEVDATA                                                                        Device data is placed in the device                                           output buffer if there is room.                                               The buffer capacity is compared to                                            a high water mark for that buffer.                                            If the buffer is near capacity and                                            the host adapter 18' to data                                                  concentrator 28 flow state is                                                 currently on, a flag is set in the                                            state tables to indicate that a                                               host xoff is pending. This state                                              information is examined and acted                                             upon by the host transmit                                                     prioritization firmware routine.                                 LHCT.sub.-- FLS                                                                            A flow control action is performed                                            according to the contents of the                                              addressed packet data field. One                                              of four flow control actions may be                                           selected by a sub-field (flow type)                                           of the data field:                                                            Flow type LHCST.sub.-- FLOWX - A flow                                         control character is transmitted to the                                       device. The flow control character is sent                                    immediately, before any data from the                                         device output buffers is sent. The flow                                       control character that is sent to the device                                  is selected by the remaining portion of the                                   data field, and is one of the characters set                                  by the LHCT.sub.-- SETXON or                                                  LHCT.sub.-- SETXOFF frames. The data                                          concentrator 28 does not return a                                             command response for this flow type.                                          Flow type LHCST.sub.-- SETLINE - The                                          state of the modem control signal output                                      lines are set according to the remainder                                      of the data field. The data concentrator                                      28 does not return a command response                                         for this flow type.                                                           Flow type LHCST.sub.-- TXDELAY - The                                          data concentrator 28 assembles a delay                                        escape sequence in the device output                                          buffer if there is room. The buffer                                           capacity is checked and acted upon as if                                      the escape sequence were device data. See                                     the description for frame-type                                                LHCT.sub.-- DEVDATA. The data                                                 concentrator 28 does not return a                                             command response for this flow type.                                          Flow type LHCST.sub.-- TXBREAK - The                                          data concentrator 28 assembles a break                                        signal escape sequence in the device                                          output buffer if there is room. The buffer                                    capacity is checked and acted upon as if                                      the escape sequence were device data. See                                     the description for frame-type                                                LHCT.sub.-- DEVDATA. The data concen-                                         trator 28 does not return a command                                           response for this flow type.                                                  Flow type LHCST.sub.-- SETAF - This type                                      sets automatic hardware flow control                                          modes of the data concentrator 28 which                                       remain in effect until changed by another                                     command, or until the data concentrator                                       28 or channel is reset. The auto DSR                                          option configures the data concentrator                                       28 to respect the DSR signal from the                                         device as a flow control signal. If this                                      state is set, the data concentrator 28 will                                   halt transmission of data to the device                                       when the signal is negated, and resume                                        transmission when this signal is asserted.                                    The auto CTS option operates similarly.                                       The auto RTS option configures the data                                       concentrator 28 to assert the RTS signal                                      when there is data available for trans-                                       mission to a device, and negate the signal                                    at all other times.                                              LHCT.sub.-- RCOND                                                                          This frame sets the status                                                    reporting conditions of the data                                              concentrator 28. The conditions                                               remain in effect until changed, or                                            until the data concentrator 28 or                                             channel is reset. The following                                               options are defined:                                                          Reporting condition LHC.sub.-- RDSR,                                          LHC.sub.-- RDCD, LHC.sub.-- RCTS - When                                       this option is selected,                                                      the data concentrator 28 will transmit a                                      modem control signal line status frame                                        to the host any time that a selected                                          input signal changes states. The modem                                        signal line status frame will always                                          reflect the state of all three signal                                         lines.                                                                        Reporting condition LHC.sub.-- FLIB - When                                    this option is selected in conjunction with                                   LHC.sub.-- RBRK, the data concentrator 28                                     will flush the host output buffer of data                                     from a particular device when a break                                         signal is detected. LHC.sub.-- RBRK must also                                 be selected for this option to take                                           effect.                                                                       Reporting condition LHC.sub.-- RBRK - When                                    this option is selected, the data                                             concentrator 28 will transmit a status                                        change frame to the host adapter 18' any                                      time a break signal from a device is                                          detected.                                                                     Reporting condition LHC.sub.-- RPE - When                                     this option is selected, the data                                             concentrator 28 will transmit a parity                                        error frame to the host adapter 18' any                                       time a parity error from a device is                                          detected. The parity error frame will                                         contain the data received from the                                            device.                                                                       Reporting condition LHC.sub.-- RFE - When                                     this option is selected, the data                                             concentrator 28 will transmit a status                                        change frame to the host any time a                                           framing error from a device is detected.                                      Reporting condition LHC.sub.-- RRXO - When                                    this option is selected, the data                                             concentrator 28 will transmit a status                                        change frame to the host any time a                                           UART receiver overrun occurs.                                    LHCT.sub.-- SBR                                                                            This frame selects the data rate for a                                        device port and selects detection modes                                       for flow control characters. If the                                           flow control character detection option                                       LHC.sub.-- XFLOW is set, the data                                             concentrator 28 will recognize the flow                                       characters as set by LHCT.sub.-- SETXON                                       and LHCT.sub.-- SETXOFF                                                       frames. If LHC.sub.-- XANY is set in                                          conjunction with LHC.sub.-- XFLOW, any                                        character is considered to be an XON                                          character.                                                       LHCT.sub.-- LPARM                                                                          This frame sets the operating parameters                                      of a device port. Character length,                                           stop bits, parity type, and parity                                            enable are the parameters which are set.                         LHCT.sub.-- SETXON                                                                         This frame sets the XON character for a                                       device port. When the flow character is                                       set, and LHC.sub.-- XFLOW is set in the                                       LHCT.sub.-- SBR frame, the data concentrator                                  28 will recognize this character as the                                       XON character.                                                   LHCT.sub.-- SETXOFF                                                                        This frame sets the XOFF character for a                                      device port. When the flow character is                                       set, and LHC.sub.-- XFLOW is set in the                                       LHCT.sub.-- SBR frame, the data concentrator                                  28 will recognize this character as the                                       XOFF character.                                                  LHCT.sub.-- CMD                                                                            This frame selects various operating                                          mode and diagnostic states. The data                                          field contains the command type to set                                        the particular state or to request the                                        particular action. The data                                                   concentrator 28 responds with a command                                       acknowledgement when the command is                                           processed. The following command types                                        are defined:                                                                  Command type LHC.sub.-- FWV - This type                                       requests the data concentrator 28 to                                          return the firmware version number. The                                       data concentrator 28 returns a firmware                                       version type frame containing the version                                     number.                                                                       Command type LHC.sub.-- WOB - This type                                       sets an operating mode in the data                                            concentrator 28 which causes it to wait                                       for a device output buffer to empty and                                       report the output buffer empty state to the                                   host adapter 18'.                                                             Command type LHC.sub.-- RLS - This type                                       requests the modem control signal line                                        status. The data concentrator 28 transmits                                    a modem control signal line status frame                                      containing the current state of all signal                                    lines.                                                                        Command type LHC.sub.-- FIB - This requests                                   that the host output buffer be flushed of                                     data for a particular device.                                                 Command type LHC.sub.-- FOB - This                                            requests that the device output buffer be                                     flushed of data.                                                              Command type LHC.sub.-- ETX - This enables                                    the transmitter for a particular channel.                                     The data concentrator 28 enables trans-                                       mitter interrupts and begins servicing                                        them. The transmitter remains enabled                                         until specifically disabled or until                                          programmed flow control disables it.                                          Command type LHC.sub.-- DTX - This                                            disables the transmitter for a particular                                     channel. It remains disabled until                                            specifically enabled.                                                         Command type LHC.sub.-- ERX - This enables                                    the receiver for a particular channel. It                                     remains enabled until specifically disabled.                                  Command type LHC.sub.-- DRX - This                                            disables the receiver for a particular                                        channel. It remains disabled until                                            specifically enabled.                                                         Command type LHC.sub.-- ETR - This enables                                    the transmitter and receiver for a                                            particular channel.                                                           Command type LHC.sub.-- DTR - This                                            disables the transmitter and receiver for a                                   particular channel.                                                           Command type LHC.sub.-- SLL - This sets                                       local loopback mode for a particular                                          channel. When this mode is set, the                                           UART of that channel will route trans-                                        mitted data back to the receiver. This                                        mode remains in effect until the channel is                                   reset or until loopback mode is reset.                                        Command type LHC.sub.-- SRL - This sets                                       remote loopback mode for a particular                                         channel. When this mode is set, the                                           UART of that channel will route received                                      device data back to the device. The mode                                      remains in effect until the channel is reset                                  or until loopback mode is reset.                                              Command type LHC.sub.-- RLL - This resets                                     local loopback mode for a channel.                                            Command type LHC.sub.-- RRL - This resets                                     remote loopback mode for a channel.                                           Command type LHC.sub.-- RSP - This resets a                                   device channel (port). The transmitter and                                    receiver are disabled, and any data in the                                    transmit or receive FIFO is flushed.                                          Command type LHC.sub.-- HLB - This sets                                       HSRT loopback mode. When this mode is                                         set, any data received from the host is sent                                  back to the host. This mode remains in                                        effect until the data concentrator 28 is                                      reset.                                                           ______________________________________                                    

When the microcontroller 74 is reset as described previously, the dataconcentrator 28 performs self test diagnostic routines and reports theresults of the diagnostic to the host adapter 18'. If any test fails, anerror is reported and the firmware then performs a jump to the beginningof its main polling loop. Once the host adapter 18' has received theerror code, it may assume that the data concentrator 28 is attempting toaccept and process data on the high speed link. The host adapter 18' maythen attempt to determine the extent of the error by issuing sequencesof diagnostic mode setting commands and data frames.

The first reset self test to be performed is a ROM checksum test,followed by a RAM test. If both the memory tests pass, the dataconcentrator 28 executes an internal loopback test for all eightchannels of the UART 78. After the data concentrator 28 executes theloopback test, it sends a frame to the host adapter 18' to indicate thetest results. The data concentrator 28 then performs a jump instructionto the beginning of its main polling loop.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction may be made without departing from the spirit of theinvention.

We claim:
 1. A communication system including data flow control,comprising:a host computer, comprising:memory for storing data, whereinsaid memory includes a data buffer for containing data to be transferredto and from said host computer, and a system bus coupled to said memory;a plurality of TTY devices, each including:a data buffer for containingdata received from and to be sent to said host computer, a portconnected to said TTY data buffer for sending data from said TTY databuffer to said host computer, and for receiving data from said hostcomputer and placing said data into said TTY data buffer, and meansconnected to said TTY data buffer and said port for detecting when thelevel of data in said TTY data buffer reaches a first predetermined highlevel and a first predetermined low level, wherein if data in said TTYdata buffer reaches said first predetermined high level, said means fordetecting temporarily stops subsequent data from being received untildata in said TTY data buffer empties to said first predetermined lowlevel; a host adapter including:a system bus connected to said hostcomputer system bus, a plurality of device output buffers, each one ofsaid plurality of device output buffers for containing data to be sentto a corresponding one of said plurality of TTY devices, acommunications port for sending and receiving data and commands, aplurality of device input buffers, each one of said plurality of deviceinput buffers for containing data sent from a corresponding one of saidplurality of TTY devices, and means connected to said host adaptersystem bus, said plurality of device input buffers, said plurality ofdevice output buffers, and said host adapter communication port fortransferring data from said data buffer of said host computer memory tosaid plurality of device output buffers, for transferring data from saidplurality of device input buffers to said data buffer of said hostcomputer memory, for transferring data from said host adaptercommunications port from said plurality of TTY devices to correspondingones of said plurality of device input buffers, for transferring datafrom said plurality of device output buffers to said host adaptercommunications port to corresponding ones of said plurality of TTYdevices, for sending a plurality of data flow commands to each of saidplurality of TTY devices, said plurality of data flow commands includinga stop-flow command for stopping a TTY device from sending subsequentdata, and a start-flow command for enabling a TTY device to sendsubsequent data, and for determining when the level of data contained ineach of said plurality of device input and output buffers reaches asecond predetermined high level and a second predetermined low level,wherein if the amount of data in one of said plurality of device inputbuffers reaches said second predetermined high level, said host adaptertransferring means sends said stop-flow command to a corresponding oneof said plurality of TTY devices, and wherein said host adaptertransferring means sends said start-flow command to said correspondingTTY device when the amount of data in said one of said plurality ofdevice input buffers is emptied below said second predetermined lowlevel; and a data concentrator, comprising:a plurality of device ports,each said device port connected to said port of a corresponding one ofsaid plurality of TTY devices for sending data to and receiving datafrom said TTY device, a plurality of device output buffers, each of saidplurality of data concentrator device output buffers for containing datato be sent to a corresponding one of said plurality of TTY devices,device buffer overflow means connected to said plurality of dataconcentrator device output buffers for detecting when the amount of datain each of said plurality of data concentrator device output buffersreaches a third predetermined high level and a third predetermined lowlevel, an input buffer for containing all data received from saidplurality of TTY devices, input buffer overflow means connected to saidinput buffer for detecting when the amount of data in said input bufferis greater than a fourth predetermined high level and less than a fourthpredetermined low level, a communications port connected to said hostadapter communications port for sending data and commands to andreceiving data and commands from said host adapter, and means connectedto said plurality of device ports, said plurality of data concentratordevice output buffers, said device buffer overflow means, said inputbuffer, said input buffer overflow means, and said data concentratorcommunications port, for transferring data for each of said plurality ofTTY devices from said data concentrator communications port to acorresponding one of said plurality of data concentrator device outputbuffers, for transferring data from each of said plurality of TTYdevices and placing data in said input buffer and for transferring datain said input buffer to said data concentrator communications port, forsending one of a plurality of commands to said host adapter through saiddata concentrator communications port, wherein said plurality ofcommands includes a flow-off command and a flow-on command, wherein ifthe level of data in one of said plurality of data concentrator deviceoutput buffers reaches said third predetermined high level, said dataconcentrator transferring means sends a flow-off command for acorresponding one of said plurality of TTY devices to said host adapterto stop said host adapter from sending data to said corresponding TTYdevice, and wherein when the amount of data in said one of saidplurality of data concentrator device output buffers empties to belowsaid third predetermined low level, said data concentrator transferringmeans sends said flow-on command for said corresponding TTY device tosaid host adapter to instruct said host adapter to start sending data tosaid corresponding TTY device, and wherein if the level of data in saidinput buffer reaches said fourth predetermined high level, said dataconcentrator transferring means stops all of said plurality of TTYdevice ports from receiving data from said plurality of TTY devices, andwherein said data concentrator transferring means enables said pluralityof TTY device ports to receive data from said plurality of TTY deviceswhen the amount of data in said input buffer is lower than said fourthpredetermined low level.
 2. The system of claim 1, wherein said hostadapter transferring means comprises a microprocessor.
 3. The system ofclaim 1, wherein said data concentrator transferring means comprises amicroprocessor.